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Poincare不等式在Poisson方程弱解中的应用1. 引言1.1 Poincare不等式的概念Poincaré不等式是数学分析中的一个重要不等式，它在函数的空间内存在一种关系，描述了函数在有界区域内的性质。
该不等式由法国数学家亨利·庇安卡雷（Henri Poincaré）于19世纪末提出，并在分析学、微分方程和椭圆偏微分方程等领域中被广泛应用。
Poincaré不等式是一个关于函数空间的不等式，用于估计函数在有界区域内的平均值与函数在该区域内的偏差之间的关系。
具体来说，对于一个有界区域Ω内的实值函数u(x)满足一定的边界条件，Poincaré不等式可以给出在这个区域内的函数的平均值距离其在各点的偏差的上界，从而揭示了函数的全局性质。
Poincaré不等式不仅在纯数学领域中有重要意义，在应用数学领域的偏微分方程、椭圆方程等问题中也有广泛的应用。
在Poisson方程的研究中，Poincaré不等式的应用可以帮助我们更好地理解问题的解的性质和存在性，进一步推动数学理论和实际问题的发展。
1.2 Poisson方程的介绍Poisson方程是一个重要的偏微分方程，通常用来描述物理学和工程学中的一些问题，如电场、流体力学和热传导等。
它的一般形式可以写成Δu = f，其中Δ是拉普拉斯算子，u是未知函数，f是给定的函数。
Poisson方程在物理学中有着广泛的应用，比如描述电势场、重力场等。
Poisson方程的解决方法很多，其中包括弱解方法。
在弱解的理论框架中，我们不要求解函数在每个点处的导数都存在，而是在某种广义意义上的意义下解决问题。
Poisson方程的弱解定义就是在某个函数空间中存在一个函数u，使得对于所有的测试函数ϕ，都有∫(∇u · ∇ϕ)dV = ∫fϕdV，其中∇表示梯度算子，f是给定的函数。
利用弱解的定义，我们可以更加灵活地处理Poisson方程的求解问题，尤其是对于不光滑的情况下。
英语长篇阅读文章对于语言学习者而言,阅读是语言输入的重要方式。
阅读策略是语言学习者为了提高阅读理解而采取的技巧和方法。
下面是店铺带来的英语长篇阅读文章，欢迎阅读!英语长篇阅读文章1科技与自然Technology that imitates natureBiomimetics: Engineers are increasingly taking a leaf out of nature's book when looking for solutions to design problems AFTER taking his dog for a walk one day in the early 1940s, George de Mestral, a Swiss inventor, became curious about the seeds of the burdock plant that had attached themselves to his clothes and to the dog's fur. Under a microscope, he looked closely at the hook-and-loop system that the seeds have evolved to hitchhike on passing animals and aid pollination, and he realised that the same approach could be used to join other things together. The result was Velcr a product that was arguably more than three billion years in the making, since that is how long the natural mechanism that inspired it took to evolve.Velcro is probably the most famous and certainly the most successful example of bio logical mimicry, or “biomimetics”. In fields from robotics to materials science, technologists are increasingly borrowing ideas from nature, and with good reason: nature's designs have, by definition, stood the test of time, so it would be foolish to ignore them. Yet transplanting natural designs into man-made technologies is still a hit-or-miss affair.Engineers depend on biologists to discover interesting mechanisms for them to exploit, says Julian Vincent, the director of the Centre for Biomimetic and Natural Technologies at theUniversity of Bath in England. So he and his colleagues have been working on a scheme to enable engineers to bypass the biologists and tap into nature's ingenuity directly, via a database of “biological patents”. The idea is that this database will let anyone search through a wide range of biological mechanisms and properties to find natural solutions to technological problems.How not to reinvent the wheelSurely human intellect, and the deliberate application of design knowledge, can devise better mechanisms than the mindless, random process of evolution? Far from it. Over billions of years of trial and error, nature has devised effective solutions to all sorts of complicated real-world problems. Take the slippery task of controlling a submersible vehicle, for example. Using propellers, it is incredibly difficult to make refined movements. But Nekton Research, a company based in Durham, North Carolina, has developed a robot fish called Madeleine that manoeuvres using fins instead.In some cases, engineers can spend decades inventing and perfecting a new technology, only to discover that nature beat them to it. The Venus flower basket, for example, a kind of deep-sea sponge, has spiny skeletal outgrowths that are remarkably similar, both in appearance and optical properties, to commercial optical fibres, notes Joanna Aizenberg, a researcher at Lucent Technology's Bell Laboratories in New Jersey. And sometimes the systems found in nature can make even the most advanced technologies look primitive by comparison, she says.The skeletons of brittlestars, which are sea creatures related to starfish and sea urchins, contain thousands of tiny lenses that collectively form a single, distributed eye. This enables brittlestarsto escape predators and distinguish between night and day. Besides having unusual optical properties and being very small—each is just one-twentieth of a millimetre in diameter—the lenses have another trick of particular relevance to micro-optical systems. Although the lenses are fixed in shape, they are connected via a network of fluid-filled channels, containing a light-absorbing pigment. The creature can vary the contrast of the lenses by controlling this fluid. The same idea can be applied in man-made lenses, says Dr Aiz enberg. “These are made from silicon and so cannot change their properties,” she says. But by copying the brittlestar's fluidic system, she has been able to make biomimetic lens arrays with the same flexibility.Another demonstration of the power of biomimetics comes from the gecko. This lizard's ability to walk up walls and along ceilings is of much interest, and not only to fans of Spider-Man. Two groups of researchers, one led by Andre Geim at Manchester University and the other by Ron Fearing at the University of California, Berkeley, have independently developed ways to copy the gecko's ability to cling to walls. The secret of the gecko's success lies in the tiny hair-like structures, called setae, that cover its feet. Instead of secreting a sticky substance, as you might expect, they owe their adhesive properties to incredibly weak intermolecular attractive forces. These van der Waals forces, as they are known, which exist between any two adjacent objects, arise between the setae and the wall to which the gecko is clinging. Normally such forces are negligible, but the setae, with their spatula-like tips, maximise the surface area in contact with the wall. The weak forces, multiplied across thousands of setae, are then sufficient to hold the lizard's weight.Both the British and American teams have shown that theintricate design of these microscopic setae can be reproduced using synthetic materials. Dr Geim calls the result “gecko tape”. The technology is still some years away from commercialisation, says Thomas Kenny of Stanford University, who is a member of Dr Fearing's group. But when it does reach the market, rather than being used to make wall-crawling gloves, it will probably be used as an alternative to Velcro, or in sticking plasters. Indeed, says Dr Kenny, it could be particularly useful in medical applications where chemical adhesives cannot be used.While it is far from obvious that geckos' feet could inspire a new kind of sticking plaster, there are some fields—such as robotics—in which borrowing designs from nature is self-evidently the sensible thing to do. The next generation of planetary exploration vehicles being designed by America's space agency, NASA, for example, will have legs rather than wheels. That is because legs can get you places that wheels cannot, says Dr Kenny. Wheels work well on flat surfaces, but are much less efficient on uneven terrain. Scientists at NASA's Ames Research Centre in Mountain View, California, are evaluating an eight-legged walking robot modelled on a scorpion, and America's Defence Advanced Research Projects Agency (DARPA) is funding research into four-legged robot dogs, with a view to applying the technology on the battlefield.Having legs is only half the story—it's how you control them that counts, says Joseph Ayers, a biologist and neurophysiologist at Northeastern University, Massachusetts. He has spent recent years developing a biomimetic robotic lobster that does not just look like a lobster but actually emulates parts of a lobster's nervous system to control its walking behaviour. The control system of the scorpion robot, which is being developed by NASAin conjunction with the University of Bremen in Germany, is also biologically inspired. Meanwhile, a Finnish technology firm, Plustech, has developed a six-legged tractor for use in forestry. Clambering over fallen logs and up steep hills, it can cross terrain that would be impassable in a wheeled vehicle.Other examples of biomimetics abound: Autotype, a materials firm, has developed a plastic film based on the complex microstructures found in moth eyes, which have evolved to collect as much light as possible without reflection. When applied to the screen of a mobile phone, the film reduces reflections and improves readability, and improves battery life since there is less need to illuminate the screen. Researchers at the University of Florida, meanwhile, have devised a coating inspired by the rough, bristly skin of sharks. It can be applied to the hulls of ships and submarines to prevent algae and barnacles from attaching themselves. At Penn State University, engineers have designed aircraft wings that can change shape in different phases of flight, just as birds' wings do. And Dr Vincent has devised a smart fabric, inspired by the way in which pine cones open and close depending on the humidity, that could be used to make clothing that adjusts to changing body temperatures and keeps the wearer cool.From hit-and-miss to point-and-clickYet despite all these successes, biomimetics still depends far too heavily on serendipity, says Dr Vincent. He estimates that there is only a 10% overlap between biological and technological mechanisms used to solve particular problems. In other words, there is still an enormous number of potentially useful mechanisms that have yet to be exploited. The problem is that the engineers looking for solutions depend on biologists havingalready found them—and the two groups move in different circles and speak very different languages. A natural mechanism or property must first be discovered by biologists, described in technological terms, and then picked up by an engineer who recognises its potential.This process is entirely the wrong way round, says Dr Vincent. “To be effective, biomimetics should be providing examples of suitable technologies from biology which fulfil the requirements of a particular engineering problem,” he explains. That is why he and his colleagues, with funding from Britain's Engineering and Physical Sciences Research Council, have spent the past three years building a database of biological tricks which engineers will be able to access to find natural solutions to their design problems. A search of the database with the keyword “propulsion”, for example, produces a range of propulsion mechanisms used by jellyfish, frogs and crustaceans.The database can also be queried using a technique developed in Russia, known as the theory of inventive problem solving, or TRIZ. In essence, this is a set of rules that breaks down a problem into smaller parts, and those parts into particular functions that must be performed by components of the solution. Usually these functions are compared against a database of engineering patents, but Dr Vincent's team have substituted their database of “biological patents” instead. Thes e are not patents in the conventional sense, of course, since the information will be available for use by anyone. By calling biomimetic tricks “biological patents”, the researchers are just emphasising that nature is, in effect, the patent holder.One way to use the system is to characterise an engineering problem in the form of a list of desirable features that thesolution ought to have, and another list of undesirable features that it ought to avoid. The database is then searched for any biological patents that meet those criteria. So, for example, searching for a means of defying gravity might produce a number of possible solutions taken from different flying creatures but described in engineering terms. “If you want flight, you don't copy a bird, but you do copy the use of wings and aerofoils,” says Dr Vincent.He hopes that the database will store more than just blueprints for biological mechanisms that can be replicated using technology. Biomimetics can help with software, as well as hardware, as the robolobster built by Dr Ayers demonstrates. Its physical design and control systems are both biologically inspired. Most current robots, in contrast, are deterministically programmed. When building a robot, the designers must anticipate every contingency of the robot's environment and tell it how to respond in each case. Animal models, however, provide a plethora of proven solutions to real-world problems that could be useful in all sorts of applications. “The set of behavioural acts that a lobster goes through when searching for food is exactly what one would want a robot to do to search for underwater mines,” says Dr Ayers. It took nature millions of years of trial and error to evolve these behaviours, he says, so it would be silly not to take advantage of them.Although Dr Vincent's database will not be capable of providing such specific results as control algorithms, it could help to identify natural systems and behaviours that might be useful to engineers. But it is still early days. So far the database contains only 2,500 patents. To make it really useful, Dr Vincent wants to collect ten times as many, a task for which he intends to ask theonline community for help. Building a repository of nature's cleverest designs, he hopes, will eventually make it easier and quicker for engineers to steal and reuse them.英语长篇阅读文章2 Lessons from a feminist paradise on Equal Pay DayOn the surface, Sweden appears to be a feminist paradise. Look at any global survey of gender equity and Sweden will be near the top. Family-friendly policies are its norm —with 16 months of paid parental leave, special protections for part-time workers, and state-subsidized preschools where, according to a government website, “gender-awareness education is increasingly common.” Due to an u nofficial quota system, women hold 45 percent of positions in the Swedish parliament. They have enjoyed the protection of government agencies with titles like the Ministry of Integration and Gender Equality and the Secretariat of Gender Research. So why are American women so far ahead of their Swedish counterparts in breaking through the glass ceiling?In a 2012 report, the World Economic Forum found that when it comes to closing the gender gap in “economic participation and opportunity,” the United States is ahead of not only Sweden but also Finland, Denmark, the Netherlands, Iceland, Germany, and the United Kingdom. Sweden’s rank in the report can largely be explained by its political quota system. Though the United States has fewer women in the workforce (68 percent compared to Sweden’s 77 percent), American women who choose to be employed are far more likely to work full-time and to hold high-level jobs as managers or professionals. Compared to their European counterparts, they own more businesses, launch more start start-ups, and more often work in traditionallymale fields. As for breaking the glass ceiling in business, American women are well in the lead, as the chart below shows.What explains the American advantage? How can it be that societies like Sweden, where gender equity is relentlessly pursued and enforced, have fewer female managers, executives, professionals, and business owners than the laissez-faire United States? A new study by Cornell economists Francine Blau and Lawrence Kahn gives an explanation.Generous parental leave policies and readily available part-time options have unintended consequences: instead of strengthening women’s attachment to the workplace, they appear to weaken it. In addition to a 16-month leave, a Swedish parent has the right to work six hours a day (for a reduced salary) until his or her child is eight years old. Mothers are far more likely than fathers to take advantage of this law. But extended leaves and part-time employment are known to be harmful to careers — for both genders. And with women a second factor comes into play: most seem to enjoy the flex-time arrangement (once known as the “mommy track”) and never find their way back to full-time or high-level employment. In sum: generous family-friendly policies do keep more women in the labor market, but they also tend to diminish their careers.According to Blau and Kahn, Swedish-style paternal leave policies and flex-time arrangements pose a second threat to women’s progress: they make employers wary of hiring wom en for full-time positions at all. Offering a job to a man is the safer bet. He is far less likely to take a year of parental leave and then return on a reduced work schedule for the next eight years.I became aware of the trials of career-focused European women a few years ago when I met a post-doctoral student fromGermany who was then a visiting fellow at Johns Hopkins. She was astonished by the professional possibilities afforded to young American women. Her best hope in Germany was a government job ––prospects for women in the private sector were dim. “In Germany,” she told me, “we have all the benefits, but employers don’t want to hire us.”Swedish economists Magnus Henrekson and Mikael Stenkula addressed the following question in their 2009 study: why are there so few female top executives in the European egalitarian welfare states? Their answer: “Broad-based welfare-state policies impede women’s representation in elite competitive positions.”It is tempting to declare the Swedish policies regressive and hail the American system as superior. But that would be shortsighted. The Swedes can certainly take a lesson from the United States and look for ways to clear a path for their high-octane female careerists. But most women are not committed careerists. When the Pew Research Center recently asked American parents to identify their "ideal" life arrangement, 47 percent of mothers said they would prefer to work part-time and 20 percent said they would prefer not to work at all. Fathers answered differently: 75 percent preferred full-time work. Some version of the Swedish system might work well for a majority of American parents, but the United States is unlikely to fully embrace the Swedish model. Still, we can learn from their experience.Despite its failure to shatter the glass ceiling, Sweden has one of the most powerful and innovative economies in the world. In its 2011-2012 survey, the World Economic Forum ranked Sweden as the world’s third most competitive economy; the UnitedStates came in fifth. Sweden, dubbed the "rockstar of the recovery" in the Washington Post, also leads the world in life satisfaction and happiness. It is a society well worth studying, and its efforts to conquer the gender gap impart a vital lesson —though not the lesson the Swedes had in mind.Sweden has gone farther than any nation on earth to integrate the sexes and to offer women the same opportunities and freedoms as men. For decades, these descendants of the Vikings have been trying to show the world that the right mix of enlightened policy, consciousness raising, and non-sexist child rearing would close the gender divide once and for all. Yet the divide persists.A 2012 press release from Statistics Sweden bears the title “Gender Equality in Sweden Treading Water” and notes: The total income from employment for all ages is lower for women than for men.One in three employed women and one in ten employed men work part-time.Women’s working time is influenced by the number and age of their children, but men’s working time is not affected by these factors.Of all employees, only 13 percent of the women and 12 percent of the men have occupations with an even distribution of the sexes.Confronted with such facts, some Swedish activists and legislators are demanding more extreme and far-reaching measures, such as replacing male and female pronouns with a neutral alternative and monitoring children more closely to correct them when they gravitate toward gendered play. When it came to light last year that mothers, far more than fathers, choseto stay home from work to care for their sick toddlers, Ulf Kristersson, minister of social security, quickly commissioned a study to determine the causes of and possible cures for this disturbing state of affairs.I have another suggestion for Kristersson and his compatriots: acknowledge the results of your own 40-year experiment. The sexes are not interchangeable. When Catherine Hakim, a sociologist at the London School of Economics, studied the preferences of women and men in Western Europe, her results matched those of the aforementioned Pew study. Women, far more than men, give priority to domestic life. The Swedes should consider the possibility that the current division of labor is not an artifact of sexism, but the triumph of liberated preference.In the 1940s, the American playwright, congresswoman, and conservative feminist Clare Boothe Luce made a prediction about what would happen to men and women under conditions of freedom:It is time to leave the question of the role of women in society up to Mother Nature — a difficult lady to fool. You have only to give women the same opportunities as men, and you will soon find out what is or is not in their nature. What is in women’s nature to do they will do, and you won’t be able to stop them. But you will also find, and so will they, that what is not in their nature, even if they are given every opportunity, they will not do, and you won’t be able to make them do it.In Luce’s day, sex-role stereotypes still powerfully limited women’s choices. More than half a century later, women in the Western democracies enjoy the equality of opportunity of which she spoke. Nowhere is this more true than Sweden. And althoughit was not the Swedes’ intention, they have demonstrated to the world what the sexes will and will not do when offered the same opportunities.Today is Equal Pay Day. But as most feminists know by now, the wage gap is largely the result of women’s vocational choices and how they prefer to balance home and family. To close the gap, it won’t be e nough to change society or reform the workplace ––it is women’s elemental preferences that will have to change. But look to Sweden: women’s preferences remain the same.Not only feminists, but also liberal and conservative policymakers should pay attentio n. Sweden is not the “tax and spend” welfare state of old ––while the rest of the world is floundering in debt, Sweden (along with its Nordic neighbors) has been downsizing, reforming entitlements, and balancing its books. The budget deficit in Sweden is about 0.2 percent of its GDP; in the United States, it’s 7 percent. But Sweden’s generous family-friendly policies remain in place. The practical, problem-solving Swedes have judged them to be a good investment. They may be right.Swedish family policies, by accommodating women’s preferences so effectively, are reducing the number of women in elite competitive positions. The Swedes will find this paradoxical and try to find solutions. Let us hope these do not include banning gender pronouns, policing childr en’s play, implementing more gender quotas, or treating women’s special attachment to home and family as a social injustice. Most mothers do not aspire to elite, competitive full-time positions: the Swedish policies have given them the freedom and opportunity to live the lives they prefer. Americans should lookpast the gender rhetoric and consider what these Scandinavians have achieved. On their way to creating a feminist paradise, the Swedes have inadvertently created a haven for normal mortals.英语长篇阅读文章3科学家告诉你：这样学才记得牢The older we get, the harder it seems to remember names, dates, facts of all kinds. It takes longer to retrieve the information we want, and it often pops right up a few minutes or hours later when we are thinking about something else. The experts say that keeping your mind sharp with games like Sudoku and crossword puzzles slows the aging process, and that may be true, but we found three other things you can do to sharpen your memory.随着年龄的增长，我们似乎越来越记不住人名、日期、还有各种事情。
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虹口区2020学年度第二学期期中学生学习能力诊断测试高三英语试卷2021.4 考生注意：1.考试时间120分钟，试卷满分140分。
2.本考试设试卷和答题纸两部分。
所有答题必须涂(选择题)或写(非选择题)在答题纸上，做在试卷上一律不得分。
3.答题前，务必在答题纸上填写准考证号和姓名，并将核对后的条形码贴在指定位置上。
I.Listening ComprehensionSection A(10分)Directions: In Section A, you will hear ten short conversations between two speakers. At the end of each conversation, a question will be asked about what was said. The conversations and the questions will be spoken only once. After you hear a conversation and the question about it, read the four possible answers on your paper, and decide which one is the best answer to the question you have heard.1. A. In the living room. B. In the bedroom.C. In the bathroom.D. In the dining room.2. A. At 8:30. B. At 9:00. C. At 9:30. D. At 10:00.3. A. To pick up the woman from the library.B. To make a copy of the schedule for his friend.C. To get a copy of the schedule for the woman.D. To find out more about the topic for the conference.4. A. She doesn’t think a lot about her studies.B. She has difficulty in doing her assignments.C. She doesn’t understand the course she is taking.D. She has no time to deal with her assignments.5. A. Language schools are being established everywhere.B. Language schools are more than other types of schools.C. Too many overseas Canadians are fond of teaching English.D. Teaching English is the biggest business throughout the world.6. A. He failed the exam. B. He studied very hard.C. The exam was too easy.D. The exam made him smile.7. A. The man can’t find a quieter place.B. The man should consider his privacy first.C. The man had better choose a low-rent apartment.D. The man is unlikely to move out of the school dormitory.8. A. The construction lasts longer than expected. B. The man usually gets up late.C. The workers drive the man crazy.D. The construction is really annoying.9. A. The man should apply for the job.B. The IT industry is booming very fast.C. The woman is lucky to have her present job.D. There are too many unemployed skilled workers.10. A. She refused the job to make ends meet.B. She refused the job because of the low salary.C. She accepted the job because of the convenient hours.D. She needed the job and would accept it despite the low salary.Section B (15分)Directions:In Section B, you will hear two short passages and one longer conversation, and you will be asked several questions on each of the passages and the conversation. The passages and the conversation will be read twice, but the questions will be spoken only once. When you hear a question, read the four possible answers on your paper and decide which one would be the best answer to the question you have heard.Questions 11 through 13 are based on the following passage.11. A. They have a short memory. B. They have language interference.C. They are lacking in language skills.D. They don’t know the language patterns.12. A. All Russian words have genders.B. Russian has fewer grammar rules.C. Russian has different sentence structures.D. Russian has an obligatory category for gender.13. A. Translators have language interference.B. Languages have different obligatory categories.C. Languages connect with the community that uses them.D. Translation is obviously a one-way street for any language.Questions 14 through 16 are based on the following passage.14. A. Because the sound of waves is too noisy.B. Because the experiment may pollute the water.C. Because the sound of waves may harm sea animals.D. Because the sea animals may disturb the experiment.15. A. To help track the sea animals being tested.B. To attract more sea animals to the testing site.C. To drive dangerous sea animals away from the testing site.D. To determine how sea animals communicate with each other.16. A. They were frightened and distressed.B. They swam away when the speaker was turned on.C. They swam closer to the speaker when the speaker was turned off.D. They swam near the speaker whether the speaker was turned on or off.Questions 17 through 20 are based on the following conversation.17. A. She wants them to lose weight, too.B. She wants them to do exercise with her.C. She wants them to stop eating unhealthy foods.D. She wants their support and help with her problems.18. A. It does harm to one’s health.B. It causes one to neglect other things.C. It doesn’t help others to increase health.D. It prevents one from achieving his or her goal.19. A. They don’t have practical goals.B. They don’t strive hard for their goals.C. They are busy and they lose focus on their goals.D. They lose focus on their goals shortly after they set them.20. A. Celebrate the success in reaching the goal.B. Set and achieve another goal of higher levels.C. Encourage close friends to set and reach their goals.D. Acknowledge the achievement and see what the next step is.II.Grammar and VocabularySection A (10分)Directions: After reading the passage below, fill in the blanks to make the passage coherent and grammatically correct. For the blanks with a given word, fill in each blank with the proper form of the given word; for the other blanks, use one word that best fits each blank.The Year in a Word or TwoCan anybody describe a year with only one word?Each December, the Oxford English Dictionary (OED) nominates (提名) a word to describe the very year (21) __________ has just passed.2020 was a very unusual year that was really worthy (22) __________ (describe) with two words. Two of the obvious words the OED suggested were “pandemic (疫情)” and “lockdown,” due to most of human activities across the globe (23) __________ (bring) to a stop by the life-threatening disease. Businesses have been closed and people have had to stay in their homes for weeks or even months on end.(24) __________ I would like to nominate “cooperation” and “hope” as my words of the year because it is supposed to be described in a more positive and (25) __________ (helpless) way, because there was a great deal of cooperation between countries and international health organizations. China took a leading role in this effort by sending masks and protective clothing to Canada, the United States and Europe, (26) __________ to countries in the Middle East and Africa. Without this cooperation, the pandemic, bad (27) __________ it has been, could have been much worse.And that brings me to my second word: hope. We are beginning to see the light at the end of the tunnel, and, with a little “hope,” we will get to see the world begin to return to normal.For young people, it was the first time that they (28) __________ (experience) something big, a worry that they shared with their families, friends and neighbors, and a suffering (29) __________ (endure) at this moment and to be endured next few weeks. And they also learned that “hope” — the belief that (30) __________ you willsee tomorrow will be better than today — is at the heart of every human being.With “cooperation” and “hope,” 2021 should make for a wonderful year.Section B (10分)Directions: Fill in each blank with a proper word chosen from the box. Each word can be used only once. Note thatWill a Robot Really Take Your Job?It is one of the most widely quoted data of recent years. No report or conference presentation on the future of work is complete without it. It has been pointed to as evidence of a(n) __31__ jobs disaster by think-tanks and government agencies. The finding that 47 percent of American jobs are at high risk of being __32__ by the mid-2030s comes from a paper written by two Oxford academics, Carl Benedikt Frey and Michael Osborne. It has since been __33__ in more than 4,000 other academic articles. Such misunderstandings reflect the polarized (两级分化的) debate __34__ the nature of automation and the future of jobs.At one extreme are the negativists. They warn of mass technological __35__ just around the corner. One advocate of this position, Martin Ford, has written two best-selling books on the dangers of unemployment caused by automation. He worries that middle-class jobs will disappear, economic __36__ will cease, and the richest people in a country could “shut themselves away in gated communities, perhaps guarded by self-directed military robots and drones.” The __37__ masses will live on a universal basic income.At the positive end of the debate, classical economists argue that in the past, new technology has always ended up creating more jobs than it has destroyed. It was several decades before industrialization led to __38__ higher wages for British workers in the early 1800s. While automation is likely to increase __39__ in the short run by pushing some people into lower-paid jobs, it eventually increases the overall size of the economic pie.Frey is often __40__ to be in the first camp. His paper simply wanted to point out that 47 percent of the current jobs in America were more likely to be affected by automation. It got more attention than they would ever have expected. In part, this is because fear sells, particularly when it is stirred up by a misunderstanding.III.Reading ComprehensionSection A (15分)Directions: For each blank in the following passage there are four words or phrases marked A. B, C and D. Fill in each blank with the word or phrase that best fits the context.Is Tap Water Safe to Drink?Tap water is not without its problems. More recently. the Michigan City of Flint has been struggling with high lead (铅) levels in its drinking water. Over the years people have witnessed major __41__ of groundwater pollution leading to unhealthy tap water.The nonprofit Environmental Working Group (EWG) detected some 260 pollutants in public water supplies in 42 states and faulted the U.S. Environmental Protection Agency (EPA) for __42__ to establish standards on so many of the pollutants — from industry, agriculture, and urban rivers — that do end up in our water.Despite these seemingly __43__ statistics, the Natural Resources Defense Council (NRDC), which has alsoconducted a series of __44__ tests on municipal (市政的) water supplies as well as bottled water, says, “In the short term, if you are an adult with no special health __45__, and you are not pregnant, then you can drink most cities’ tap water without having to worry.” This is because most of the pollutants in public water supplies exist at such small concentrations that most people would have to __46__ very large quantities for health problems to occur.__47__, look at your water bottles carefully. It’s common for them to list the source as “municipal”, which means you paid for what __48__ is bottled tap water. “Pregnant women, young children, the elderly, people with chronic illnesses and those with weakened immune systems can be vulnerable(脆弱的) to the risks posed by polluted water.” NRDC does __49__ however. The group suggests that anyone who may be at risk obtain a copy of their city’s annual water quality report and review it with their __50__.As for bottled water, 25 to 30 percent of it comes straight from municipal tap water systems, despite the pretty nature __51__ on the bottles that imply otherwise. Some of that water goes through additional filtering (过滤), but some does not. What’s more, bottled water is required to be __52__ less frequently than tap water for bacteria and chemical pollutants, and U.S. Food and Drug Administration bottled water rules allow for some pollution by E. coli (大肠杆菌), __ 53__ to EPA tap water rules that prohibit any such pollution.Also, NRDC found that there are no __54__ for bottled water to be disinfected or tested for parasites(寄生虫), unlike more strict EPA rules regulating tap water. This leaves the possibility __55__ says NRDC, that some bottled water may present similar health threats to those with weakened immune systems.41. A. cases B. patterns C. models D. modes42. A. managing B. failing C. competing D. skipping43. A. endless B. appealing C. alarming D. meaningless44. A. expansive B. expensive C. aggressive D. extensive45. A. motivations B. focuses C. conditions D. proportions46. A. turn in B. give away C. take up D. take in47. A. To start with B. After all C. In addition D. On the whole48. A. necessarily B. essentially C. accordingly D. dramatically49. A. recommend B. caution C. justify D. urge50. A. supervisor B. secretary C. authority D. physician51. A. scenes B. brands C. data D. prints52. A. questioned B. produced C. tested D. advertised53. A. contrary B. similar C. relative D. parallel54. A. requirements B. suggestions C. permissions D. approvals55. A. real B. minor C. uncertain D. openSection B (22分)Directions: Read the following three passages. Each passage is followed by several questions or unfinished statements. For each of them there are four choices marked A. B. C and D. Choose the one that fits best according to the information given in the passage you have just read.(A)Each generation — from Baby Boomers to Generation X to Millennials — has its own set of values andcharacteristics. But one thing common to all generations is that they are suffering from stress. In a recent poll by the American Psychological Association (APA), all age groups now report higher levels of stress than in the past. Baby Boomers (those born roughly between 1946 and 1964, and who are now moving into their retirement years) said that they are stressed about earnings and health issues. Gen Xers (born roughly between 1965 and 1980) are concerned about work, income, and job stability. However, Millennials (born roughly between 1981 and 2004) are turning out to be the most stressed-out of all the generations. Poll results indicate that stress levels for these younger respondents are significantly above average. So what’s worrying the Millennials?Millennials are the first generation to grow up with computers in the home and the classroom. Due to the rise of modern technology and social media, they are constantly showered with information. Over time, this information overload can become too much to handle and can result in stress, which in turn can cause serious physical, psychological, and emotional problems. Another contributing factor, according to author Michael D. Hais, is that many Millennials have lived sheltered lives due to overprotective parents. These young adults lack problem-solving skills and may struggle with fear of failure once they leave home. Making matters worse, the 2008 economic depression occurred when many Millennials were graduating from high school or college. The resulting economic slowdown reduced the number of available jobs for graduates. Sure enough, in the APA poll, Millennials said that work, money, relationships, family responsibilities, and the economy are the main stressors in their lives.However, the poll results may be a bit misleading as they don’t take into account public attitudes toward stress and mental illness. Ronald Kessler of Harvard Medical School, who has studied the prevalence (流行) of mental disorders in the U.S., points out that changes in social attitudes have helped reduce the stigma attached to mental illness over the years, For example, the creation of health-related television programming and specialty magazines such as Psychology Today have contributed to greater public awareness of mental health issues. According to psychologists, younger people now are more willing to admit to them that they are under stress than in the past. “There is not a lot of evidence of true prevalence having gone up,” Kessler says. “It looks like younger people are in worse shape, but unfortunately, we just don’t know.”56.It can be learned from the results of the APA poll that __________.A. All age groups are experiencing more stress than beforeB. Millennials have more stress than before, but other age groups aren’tC. Compared with other age groups, Millennials have more overprotective parentsD. Stress levels are down for all age groups, but they’re down the most for Millennials57.__________ is a cause of worry across all generations.A. HealthB. OccupationC. MoneyD. Weak economy58.What’s the possible meaning of the underlined word “stigma” in the last paragraph?A. Behavior.B. Shame.C. Symptom.D. Complexity.59.According to the passage, what can be inferred about the young people today compared to the past?A. They fail to keep good body shape.B. They are more interested in health-related media.C. They are more willing and courageous to face up to their fault.D. They are more willing to seek professional help to deal with stress.(B)Bacteria are the one of the main food poisoners. So, to get food on the table safely, you need to know and follow the rules for food care.KEEP FOOD HOTHigh food temperatures (165ºF to 212ºF) reached in boiling, baking, frying, and roasting kill most food poisoning bacteria. If you want to delay serving cooked food, though, you have to keep it at a holding temperature — roughly 140ºF to 165ºF. Steam tables and chafing dishes are designed to maintain holding temperatures. But they don’t always keep food hot enough. So it’s not wise to leave hot food out more than 2 hours.When cooked food is left out unheated, the possibility of bacterial growth is greater, since the food quickly drops to room temperature where food poisoners multiply. To serve hot foods safely — particularly meat and poultry, which are highly affected by food poisoning — follow these rules:Cook thoroughly — Cook meat and poultry to the “doneness” temperatures given in the above chart. To make sure that meat and poultry are cooked all the way through, use a meat thermometer (温度计). Insert the tip into the thickest part of the meat, avoiding fat or bone.Don’t interrupt cooking — Cook meat and poultry completely at one time. Partial cooking may encourage bacterial growth before cooking is complete.Cooking frozen food — Allow frozen fond more time to cook — generally 1.5 times the period required for food that has been thawed (解冻).60.At what temperature do bacteria grow the fastest?A. 40ºC ~ 60ºC.B. 60ºF ~ 125ºF.C. 40ºC ~ 140ºC.D. 125ºF ~ 140ºF.61.To keep food safe, it is suggested that __________.A. hot food should not be left out over two hoursB. frozen food should be thawed with a microwave ovenC. raw ham should be cooked to the “doneness” temperature at 71ºFD. steam tables and chafing dishes should be used to keep food hot enough62.What can be learned according to the guideline?A. The only way to ensure food safety is to keep it hot enough.B. High food temperatures above 74ºC destroy most poisoning bacteria.C. Compared with goose, ground beef should be cooked to higher temperatures.D. Insert the tip of a thermometer into the thickest part of the fat to control the temperature.(C)“A writer’s job is to tell the truth,” said Hemingway in 1942. No other writer of our time had so fiercely stated, or so consistently(一贯地) illustrated the writer’s duty to speak truly. His standard of truth-telling remained, moreover, so high and so strict that he was ordinarily unwilling to admit secondary evidence, whether literary evidence or evidence picked up from other sources than his own experience. “I only know what I have seen”, was a statement which came often to his lips and pen. What he had personally done, or what he knew unforgettably by having gone through one version of it, was what he was interested in telling about.The primary intention of his writing, from first to last, was to seize and project for the reader what he often called “the way it was”. This is a characteristically simple phrase for a concept of extraordinary complexity, and Hemingway’s concept of its meaning subtly (微妙地) changed several times in the course of his career — always in the direction of greater complexity. At the core of the concept, however, one can invariably recognize the operation of three instruments of beauty appreciation: the sense of place, the sense of fact, and the sense of scene.The first of these, obviously a strong passion with Hemingway, is the sense of place. “Unless you have geography, background,” he once told George Antheil, “you have nothing.” You have, that is to say, a dramatic vacuum. Few writers have been more place-conscious. Few have so carefully charted out the geographical ground work of their novels while managing to keep background so unnoticeable. Few, accordingly, have been able to record more economically and graphically the way it is when you watch the bulls running through the streets of Pamplona, Spain towards the bull-ring.“When I woke it was the sound of the rocket exploding that announced the release of the bulls. Down below the narrow street was empty. All the balconies were crowded with people. Suddenly a crowd came down the street. They were all running, packed close together. They passed along and up the street toward the bull-ring and behind them came more men running faster, and then some stragglers (落后者) who were really running. Behind them was a little bare space, and then the bulls tossing their heads up and down. It all went out of sight around the comer. One man fell, rolled to the gutter (排水沟), and lay quiet. But the bulls went right on and did not notice him. They were all running together.”63.What’s the main idea of the first two paragraphs?A. Hemingway’s writing began from reality and then he would let his mind wander.B. Hemingway’s primary purpose in writing was to report faithfully reality as he experienced it.C. Hemingway’s writing reflects his preference for a simple story that the reader would thoroughly enjoy.D. Hemingway would construct a story that would reflect truths that were not particular to a specific historicalperiod.64.It can be inferred from the passage that Hemingway preferred __________ as the sources for his work.A. Stories that he had experienced rather than read aboutB. Stories that he had read about in newspapers or other sourcesC. Stories that he had heard from friends or chance acquaintancesD. Stories that came to him in periods of deep thinking or in dreams65.The author calls “the way it was” a “characteristically simple phrase for a concept of extraordinary complexity”because __________.A. it shows how Hemingway understated complex issues in his booksB. it reflects Hemingway’s talent for making ordinary events difficult to understandC. Hemingway’s obsession for geographic details overshadowed the dramatic element of his storiesD. the relationship between simplicity and complexity reflected the relationship between the style and contentof Hemingway’s writing66.Why does the author include an except (节选) from The Sun Also Rises in the last paragraph?A. To vividly illustrate how exciting the bull run is.B. To demonstrate that all kinds of runners can take part in the bull run.C. To show Hemingway’s delicate description of the background of the bull run.D. To place greater emphasis on the importance of geography, economically speaking.Section C (8分)Directions: Read the passage carefully. Fill in each blank with a proper sentence given in the box. Each sentenceBenefits of Cooperative LearningIn the classroom teachers should deliberately create opportunities for students to cooperate with each other, share responsibilities, solve problems, and control conflict.(67)__________ Cooperative learning activities require students to work together in small groups to completea project or activity, operating as a team to help each other succeed.You may be wondering what benefits students gain from cooperative learning. The answer is many! Cooperative learning, of course, teaches a number of social and emotional skills, but it also gives students the opportunity to learn from each other. The following skills that are developed through regular and effective cooperative learning are just a few of many.In order for a cooperative learning group to succeed, individuals within the group need to show leadershipabilities. Without this, the group cannot move forward without a teacher. Natural leaders become quickly evident in small groups, but most students don’t naturally want to lead. (68)__________Also, effective teamwork requires good communication and commitment. All members of a cooperative learning group have to learn to speak productively with one another to stay on track. By teaching students to share confidently, listen carefully, and speak clearly, they learn to value the input of their teammates and the quality of their work soars.Conflicts are bound to arise in any group setting. (69)__________ Give students space to try and work out their issues for themselves before stepping in.There are many decisions to be made in a cooperative environment. Encourage students to think as a team to make joint decisions by first having them come up with a team name. (70)__________ Make sure that each student has their own responsibilities in cooperative learning groups. Much like leadership skills, decision-making skills cannot be developed if students are not regularly practicing them.IV.Summary Writing (10分)Directions: Read the following passage. Summarize the main idea and the main point(s) of the passage in no more than 60 words. Use your own words as far as possible.71.New Testing Method in Era of Online LearningIn research published today in Science of Learning, engineers from Rensselaer Polytechnic Institute demonstrate how a testing strategy they call “distanced online testing” can effectively reduce students’ ability to receive help from one another in order to score higher on a test taken at individual homes during social distancing.“Often in remote online exams, students can talk over the phone or internet to discuss answers,” said Ge Wang, a chair professor of biomedical engineering at Rensselaer and the corresponding author on this paper. “The key idea of our method is to minimize this chance via discrete optimization (离散优化) aided by knowledge of a student’s competencies.”When a distanced online test is performed, students receive the same questions, but at varying times depending on their skill level. For instance, students of highest mastery levels receive each question after other groups of students have already answered those questions. This approach, Wang said, reduces the motivator for students to receive help from those who have more mastery of the material. In order to determine the order of each student’s questions, their competence levels are estimated using their grade point averages, SAT scores, or midterm scores, depending on what is available at a specific point in the semester.According to statistical tests and post-exam surveys, this method reduced the points gained through cheating by orders of magnitude (数量级) when compared to conventional exam methods. As an added benefit, Wang said, when students knew collusion would not be possible, they were more motivated to study class material. Wang and his colleagues hope to share this innovation of teaching methods and theory beyond the Rensselaer campus.“We plan to develop a good platform so that others can easily use this method,” said Wang, a member of the Center for Biotechnology and Interdisciplinary Studies at Rensselaer.V.Translation (15分)Directions: Translate the following sentences into English, using the words given in the brackets.。
2021年12月四级考试预测押题卷（一）Part I Writing(30minutes)Directions:For this part,you are allowed30minutes to write a letter to offer your suggestions to your cousin who sought your advice on how to make his resume distinctive.You should write at least120words but no more than 180words.Part II Listening Comprehension(25minutes)Section ADirections:In this section,you will hear three news reports.At the end of each conversation,you will hear four questions.Both the news report and the questions will be spoken only once.After you hear a question,you must choose the best answer from the four choices marked A),B),C)and D).Then mark the corresponding letter on Answer Sheet1with a single line through the centre.Questions1and2are based on the news report you have just heard.1.A)Two.B)Three.C)Four.D)Five.2.A)He called the police after the accident.B)He broke his arm in the accident.C)He was caught taking drugs.D)He was arrested by the police.Questions3and4are based on the news report you have just heard.3.A)A cure to brain cancer.B)A new surgical instrument.C)A pen that can identify cancerous tissue.D)A new drug that can eliminate cancerous tissue.4.A)Finding the border between the cancerous and normal tissue.B)Identifying the accuracy rate of the new device.C)Improving their speed of removing a tumour.D)Using the new device in brain surgery.Questions5to7are based on the news report you have just heard.5.A)To collect scientific data on it.C)To take photos of the storm on it.B)To monitor the storm on it.D)To investigate its environment.6.A)It has lasted for nearly350years.B)It has lasted for more that350months.C)It seems to be getting smaller.D)It seems to be getting larger.7.A)What initially caused the storm.C)What is the impact of the storm.B)What is underneath the storm.D)What makes the storm last for so long.Section BDirections：In this section,you will hear two long conversations.At the end of each conversation,you will hear four questions.Both the conversation and the questions will be spoken only once.After you hear a question,you must choose the best answer from the four choices marked A),B),C),and D).Then mark the corresponding letter on Answer Sheet1with a single line through the centre.Questions8to11are based on the conversation you have just heard.8.A)It’s for disabled adults.B)It’s in a sports centre.C)It’s rewarding and challenging.D)It’s compulsive in her community.9.A)The skills they need.B)The products they have.C)The market they target.D)The language they require.10.A)Diversify markets and sales strategies.B)Reduce costs and jobs.C)Learn from other companies.D)Listen to the opinions of experts.11.A)The salary and the workload.B)The office hour and the penalty system.C)The welfare and the holiday system.D)The ethical policy and the carbon footprint.Questions12to15are based on the conversation you have just heard.12.A)Double-decker buses.B)The traffic in London.C)Bus routes.D)Travels in Britain.13.A)It has no windows.B)People get onto it at the front.C)It has two carriages.D)It is open at the back.14.A)Uncomfortable.B)Noisy.C)Dangerous.D)Shabby.15.A)Bendy buses can help reduce the traffic jam.B)Bendy buses are more environmentally friendly.C)Bendy buses are convenient for people in wheelchairs.D)Bendy buses are more popular among tourists.Section CDirections:In this section,you will hear three passages.At the end of each passage,you will hear some questions. Both the passage and the questions will be spoken only once.After you hear a question,you must choose the best answer from the four choices marked A),B),C)and D).Then mark the corresponding letter on Answer Sheet1 with a single line through the centre.Questions16to18are based on the passage you have just heard.16.A)They had four toes.B)They were not as big as dogs.C)They lived in South America.D)They lived in thick forests.17.A)They had long legs and a long tail.B)They were smaller and had front eyes.C)They began to eat grass as well as fruit.D)They were bigger and had long legs.18.A)They evolved into donkeys in Asia and Africa.B)They used their long legs to run south to South Africa.C)They began to eat apples on the North American plains.D)They preferred grass to fruit and vegetables.Questions19to21are based on the passage you have just heard.19.A)Being rejected by friends and teachers.B)Staying away from his native land.C)Adapting to new study expectations.D)Keeping a balance between study and job.20.A)Talking with older brothers or sisters.C)Starting a conversation with close friends.B)Having a casual talk with a college student.D)Playing with friends on the same sports team.21.A)Follow traditions of with a college student.C)Respect the customs of different colleges.B)Take part in as many activities as possible.D)Take others’advice as reference only.Questions22to25are based on the passage you have just heard.22.A)They tend to harm wildlife.C)They are thrown away everywhere.B)They are hardly recyclable.D)They are made from useless materials.23.A)It is fatal.B)It is weird.C)It is very serious.D)It is complicated.24.A)The sea creatures that have taken in then are consumed by humans.B)The ocean’s ecology has been polluted and affected humans.C)Humans eat the seabirds that have swallowed plastic particles.D)Humans consume the fish that have eaten sea creatures with them.25.A)Its use has been drastically reduced.C)Most products use natural materials.B)It is still an indispensable material.D)The use of plastic items will be charged.PartⅢReading Comprehension(40minutes)Section ADirections:In this section,there is a passage with ten blanks.You are required to select one word for each blank from a list of choices given in a word bank following the passage.Read the passage through carefully before making your choices.Each choice in the bank is identified by a letter.Please mark the corresponding letter for each item on Answer Sheet2with a single line through the centre.You may not use any of the words in the bank more than once.Questions26to35are based on the following passage.A third of the planet’s land is severely degraded and fertile soil is being lost at the rate of24bn tonnes a year, according to a new United Nations-backed study that calls for a shift away from destructively intensive agriculture, The alarming____26____,which is forecast to continue as demand for food and productive land increases,will ass to the risks of conflicts unless____27____actions are implemented,warns the institution behind the report.“As the ready supply of healthy and productive land dries up and the population grows,competition is ___28___for land within countries and globally,”said executive secretary of the UN Convention to Combat Desertification(UNCCD)at the launch of the Global Land Outlook.“To___29____the losses,the outlook suggests it is in all our interests to step back and rethink how we are managing the pressures and the competition.”The Global Land Outlook is____30____as the most comprehensive study of its type,mapping the interlinked impacts of urbanization,climate change,erosion and forest loss.But the biggest factor is the___31___of industrial farming.Heavy tilling,multiple harvests ans___32____use of agrochemicals have increased yields at the____33____of long-term sustainability.If the past20years,agricultural production has increased threefold and the amount of irrigated land has doubled,notes a paper in the outlook by the Joint Research Centre(JRC)of the European commission.Over time,however,this___34___fertility and can lead to abandonment of land and ___35___desertification.A)absorb I)limitedB)abundant J)minimizeC)billed K)occasionallyD)decline L)optimizesE)diminishes M)rateF)expansion N)remedialG)expense O)ultimatelyH)intensifyingSection BDirections:In this section,you are going to read a passage with ten statements attached to it.Each statement contains information given in one of the paragraphs.Identify the paragraph from which the information is derived. You may choose a paragraph more than once.Each paragraph is marked with a letter.Answer the questions by marking the corresponding letter on Answer Sheet2.Take Naps at Work.Apologize to No One[A]In the past two weeks I’ve taken three naps at work,a total of an hour or so of shut-eye while on the clock.And I have no shame or uncertainty about doing it.I couldn’t feel better about it,and my productivity reflects it,too.[B]Sleeping on the job is one of those workplace taboos-like leaving your desk for lunch or taking an afternoon walk-that we’re taught to look down on.If someone naps at2p.m.while the rest of us furiously write memos and respond to emails,surely it must mean they’re slacking off(偷懒).Or so the assumption goes.[C]Restfulness and recharging can take a back seat to the perception and appearance of productivity.It’s easier to stay on a virtual hamster(仓鼠)wheel of activity by immediately responding to every email than it is to measure aggregate productivity over a greater period of time.But a growing field of occupational and psychological research is building the case for restfulness in pursuit of greater productivity.[D]Companies are suffering from tremendous productivity problems because people are stressed out and not recovering from the workday,said Josh Bersin,Principal and Founder of Bersin by Deloitte.“They’re beginning to realize that this is their problem,and they can’t just say to people,‘Here’s a work-life balance course,go teach yourself how to manage your inbox,’”Mr.Bersin said.“It’s way more complicated than that.”[E]To be sure,the ability to nap at work is far from widespread,experts said.Few among us have the luxury of being able to step away for a half-hour snoozefest.But lunch hours and coffee breaks can be great times to duck out,and your increased productivity and alertness will be all the evidence you need to make your case to inquiring bosses.[F]In an ideal world,we’d all solve this problem by unplugging early and getting a good night’s sleep. Here’s our guide on how to do just that.But the next best thing is stealing away for a quick power nap when you’re dragging after lunch.[G]In a study published in Nature Neuroscience,researchers tested subjects on their perceptual performance four times throughout the day.Performance deteriorated with each test,but subjects who took a30-minute nap between tests stopped the deterioration in performance,and those who took a60-minute nap even reversed it.[H]“Naps had the same magnitude of benefits as full nights of sleep if they had a quality of nap.”said Sara Mednick,a co-author of the study and associate professor of psychology at the University of California,Riverside.[I]Dr.Mednick,a sleep researcher and the author of Take a Nap!Change Your Life,said daytime napping can have many of the benefits of overnight sleep,and different types of naps offer specific benefits.[J]For example,Dr.Mednick said a20-to60-minute nap might help with memorization and learning specific bits of information.It’s just long enough to enter stage-two sleep,or non-rapid eye movement(R.E.M.)sleep.[K]After60minutes,you start getting into R.E.M.sleep,most often associated with that deep,dreaming state we all enjoy at night R.E.M.sleep can improve creativity,perceptual processing and highly associativethinking,which allows you to make connections between disparate ideas,Dr.Mednick said.Beyond that,your best bet is a90-minute nap,which will give you a full sleep cycle.[L]Any nap,however,can help with alertness and perception and cut through the general fog that creeps in during the day,experts said.[M]So how did we even arrive at this point where aptitude is inextricably tied(紧密相连)to working long, concentrated hours?Blame technology,but think broader than smartphones and laptops;the real issue is that tech has enabled us to be available at all times.[N]“We went through a period where people were in denial and business leaders were ignoring it,”Mr. Bersin said.“They were assuming that if we give people more tools,more emails,more Slack,more chatter,and we’ll just assume they can figure out how to deal with it all.And I think they’ve woken up to the fact that this is a big problem,and it is affecting productivity,engagement,health,safety,wellness and all sorts of things.”[O]It isn’t just office workers who can benefit from an afternoon siesta(午睡).A2015study published in Current Biology looked at the at the sleeping habits of three hunter-gatherer preindustrial societies in Tanzania, Namibia and Bolivia.[P]“They’re active in the morning,then they get in the shade under the trees and have a sort of quiet time, but they’re not generally napping,”said Jerome Siegel,professor of psychiatry and biobehavioral sciences,and director of the U.C.L.A.Center for Sleep Research,a co-author of the study.“Then they do some work and go to sleep,and they sleep through the night.”[Q]Still,Mr.Siegel said,“the only genuine way to solve daytime sleepiness and fatigue starts the night before with a solid night’s sleep.”The real Holy Grail of restfulness is a regular sleep schedule with ideally seven or eight hours of sleep each night,which experts say is optimal.[R]“Daytime napping certainly does increase alertness,”Mr.Siegel said.“But it’s not as simple as going to the gas station and filling the tank.”[S]He also advises avoiding caffeine late in the day and waking around the same time every morning,even if you can’t get to sleep at the same time every night,This helps acclimate(使适应)your body to your regular wake-up time,regardless of how much sleep you got the night before.[T]So if you’ve made it this far and you’re interested in giving workday naps a try(or just starting to nod off),here’s a quick guide to the perfect nap;Find a quiet,unoccupied space where you won’t be disturbed.Try to make your area as dim as possible(or invest in a sleep mask you can keep in the office).Earplugs might help.too.Aim for around20minutes.Any longer than that and you’re likely to wake up with sleep inertia(睡眠惰性)，which will leave you even groggier(头脑昏沉的)than before.36.Participants’perceptual performance became better after sleeping one hour between tests in an article inNature Neuroscience.37.Jerome Siegel found that only by sleeping soundly through the previous night could people tackle theirweariness during the day.38.Our talent is closely bound to working with concentration for long periods of time because technologymakes us accessible24/7.39.Taking a nap at work is normally regarded as laziness that should be held in contempt and avoided inworkplace.40.Between20to60minutes,people can get into non-REM sleep which may improve memory and learningability according to Dr.Mednick.41.People can doze off at lunch and coffee breaks and defended themselves by saying their improvedproductivity and alertness when bosses investigated their whereabouts.42.The author’s tips on taking a perfect nap involve sleeping place,environment and duration.43.The author believes business leaders are aware that availability at any time due to technology has negativeeffects on every aspect of people’s life.44.The optimal length of a nap was an hour and a half so that people could go through a complete sleep cycle.45.Josh Bersin mentioned the cause of companies’big productivity problems and the solution which needsmore that just employees’efforts.Section CDirections:There are2passages in this section.Each passage is followed by some questions or unfinished statements.For each of them there are four choices marked A),B),C)and D).You should decide on the best choice and mark the corresponding letter on Answer Sheet2with a single line through the centre.Passage OneQuestions46to50are based on the following passage.Every office worker hates meetings.But it’s a strange sort of hate,similar to the hatred of Londoners for the Northern Line,or New Yorkers for tourists who walk too slowly:the dislike is real,yet if the despised thing were to vanish,it’d be like surrendering a piece of your soul.When researchers probed into why people put up with the strain that meetings place on their time and sanity, they found something-those who resent and dread meetings the moat also defend them as a“necessary evil”, sometimes with great passion.True,research suggests that meetings take up vastly more of the average manager’s time than they used to.True,done badly,they’re associated with lower levels of innovation and employee wellbeing(幸福).But that’s just office life,right?It’s not supposed to be fun.That’s why they call it work.Underlying(引起)this attitude is an assumption that’s drummed into us not just as workers but as children, parents and romantic partners;that more communication is always a good thing.So suggestions abound for(大量存在)communicating better in meetings-for example,hold them standing up,so speakers will come to the point more quickly.But even when some companies consider abolishing meetings entirely,the principle that more communication is better isn’t questioned.If anything,it’s reinforced when such firms introduce“flat”management structures,with bosses always available to everyone,plus plenty of electronic distraction.In fact,constant connectivity is disastrous for both job satisfaction and the bottom line.And anyway,once you give it three seconds’thought,isn’t it cleat that more communication frequently isn’t a good thing?Often,the difference between a successful marriage and a second-rate one consists of leaving about three or four things a day unsaid.At work,it’s surely many more than four,though for a different reason;office communication comes at the cost of precisely the kind of focus that’s essential to good work.Yet we’re so accustomed to seeing talking as a source of solutions-for resolving conflicts or finding new ideas-that it’s hard to see when it is the problem.46.What does the author say about meetings?A)Londoners hate them as well as the Northern Line.B)They can help to keep workers’physical and spiritual health.C)Workers might be reluctant to give up them completely.D)New Yorkers dislike meetings more than Londoners.47.What did researchers find about people’s attitude towards meeting?A.Their attitude and behavior are paradoxical.B)People who hate meetings the most are senior insane.C)Those who like meetings might be considered insane.D)More meetings are regarded as a sign of less innovation.48.Why do people think that more communication is always a good thing?A)Because the concept is firmly believed by workers.B)Because everyone loves to communicate with others.C)Because the idea has been instilled into people’s mind.D)Because communication is vital for building relationships.49.What does the author think of the“flat”management structure?A)It forces bosses to frequently contact their employees.B)It helps to soften employees’bottom line of work.C)It is definitely a disaster to employees’job satisfaction.D)It strengthens people’s deeply-rooted notion of communication.50.What is the author’s argument about office communication?A)It is an effective way to solve office conflicts.B)It affects work efficiency in a negative way.C)It should come to a halt at intervals.D)It is useful for workers to find new ideas.Passage TwoQuestions51to55are based on the following passage.The Internet has enabled the spread of information at lightning speed.This information revolution has created tremendous business opportunities for online publishers,but not all of them maintain proper quality-control mechanisms to ensure that only good information is being shared.Instead,many publishers aim simply to make money by whatever means possible,with no regard for the implications for society at large.When selfish publishers set up shops online,the primary goal is to publish as much as possible,often at the cost of quality.In this respect,many publishers start numerous online journals focused on overlapping(重叠的)disciplines—to increase their total number of published papers—and hire young business managers who do not have any experience in either science or publishing.In some cases,online publishers even give up peer review, while still presenting themselves as scientific journals—deception designed to take advantage of scientists who simply want to share their research.If publishers structure their business to make more revenue,it often does harm to their products.When publishers start journals with overlapping domains,in combination with the pressure to publish more studies,this could promote the publication of marginal or even questionable articles.Moreover,publishers with multiple overlapping journals and journals with very narrow specialties(专业)increase the demands on the time and efforts of willing reviewers.With the fact that reviewers are generally not compensated for their time and effort,journal editors are often unable to find enough reviewers to keep up with the increased publication rate.To improve the situation and increase the trust in scientific community,the pressure to publish must be reduced.Funding and promotion decisions should not be based on the number of publications,but on the quality of those publications and a researcher’s long-term productivity and instructions.And that’s just the start.We need additional mechanisms,such as Beall’s list of predatory(掠夺的)publishers, to alert scientists to fake journals and fake articles.In addition,the price for online publication must be controlled and a mechanism must be put in place to honor and reward hard-working reviewers.51.What does the author think of online publishers?A)A small proportion of them can guarantee their publishing quality.B)They have lots of opportunities to renovate their business models.C)Many of them tend to try every means to make a buck.D)Social impact is their first priority when publishing books.52.It can be inferred from the second paragraph that______.A)peer review generally is a criterion to identify academic journalsB)researchers focus their research on the combination of disciplinesC)scientists care about their publications rather than researchD)young business managers are willing to face new challenges53.Why can’t publishers find enough reviewers to review papers?A)Reviewers are pressed for time when reviewing articles.B)Reviewers’gains can’t make up for what they have done.C)Publishers may compel reviewers to accept marginal articles.D)Publishers urge reviewers to increase publication rate rapidly.54.What is the author’s suggestion for online publication?A)More weight should be put on the quantity of publications.B)It is worthwhile to reward diligent reviewers for their effort.C)Fake journals should be reported to a regulatory organization.D)The price of online publication should be lowered greatly.55.What is the main idea of this passage?A)Online publishers should take measures to fight against fake scientific journals.B)Online publishers are pursuing their work efficiency at the cost of quality.C)Online publishers business models are quite likely to harm their publications.D)Online publishers are sacrificing the quality of research articles to make money.PartⅣTranslation(30minutes)Directions:For this part,you are allowed30minutes to translate a passage from Chinese intoEnglish.You should write your answer on Answer Sheet2.春节是中国的传统节日，相当于美国的圣诞节。
高斯朴素贝叶斯训练集精确度的英语Gaussian Naive Bayes (GNB) is a popular machine learning algorithm used for classification tasks. It is particularly well-suited for text classification, spam filtering, and recommendation systems. However, like any other machine learning algorithm, GNB's performance heavily relies on the quality of the training data. In this essay, we will delve into the factors that affect the training set accuracy of Gaussian Naive Bayes and explore potential solutions to improve its performance.One of the key factors that influence the training set accuracy of GNB is the quality and quantity of the training data. In order for the algorithm to make accurate predictions, it needs to be trained on a diverse and representative dataset. If the training set is too small or biased, the model may not generalize well to new, unseen data. This can result in low training set accuracy and poor performance in real-world applications. Therefore, it is crucial to ensure that the training data is comprehensive and well-balanced across different classes.Another factor that can impact the training set accuracy of GNB is the presence of irrelevant or noisy features in the dataset. When the input features contain irrelevant information or noise, it can hinder the algorithm's ability to identify meaningful patterns and make accurate predictions. To address this issue, feature selection and feature engineering techniques can be employed to filter out irrelevant features and enhance the discriminative power of the model. Byselecting the most informative features and transforming them appropriately, we can improve the training set accuracy of GNB.Furthermore, the assumption of feature independence in Gaussian Naive Bayes can also affect its training set accuracy. Although the 'naive' assumption of feature independence simplifies the model and makes it computationally efficient, it may not hold true in real-world datasets where features are often correlated. When features are not independent, it can lead to biased probability estimates and suboptimal performance. To mitigate this issue, techniques such as feature extraction and dimensionality reduction can be employed to decorrelate the input features and improve the training set accuracy of GNB.In addition to the aforementioned factors, the choice of hyperparameters and model tuning can also impact the training set accuracy of GNB. Hyperparameters such as the smoothing parameter (alpha) and the covariance type in the Gaussian distribution can significantly influence the model's performance. Therefore, it is important to carefully tune these hyperparameters through cross-validation andgrid search to optimize the training set accuracy of GNB. By selecting the appropriate hyperparameters, we can ensure that the model is well-calibrated and achieves high accuracy on the training set.Despite the challenges and limitations associated with GNB, there are several strategies that can be employed to improve its training set accuracy. By curating a high-quality training dataset, performing feature selection and engineering, addressing feature independence assumptions, and tuning model hyperparameters, we can enhance the performance of GNB and achieve higher training set accuracy. Furthermore, it is important to continuously evaluate and validate the model on unseen data to ensure that it generalizes well and performs robustly in real-world scenarios. By addressing these factors and adopting best practices in model training and evaluation, we can maximize the training set accuracy of Gaussian Naive Bayes and unleash its full potential in various applications.。
带狄利克雷边界条件的小初值耗散半线性波动方程外问题解的破裂及生命跨度估计徐根海;吴邦【摘要】研究在高维外区域上带狄利克雷边界条件的耗散半线性波动方程ut-Δu+ut=|u|p的初边值问题.证明了无论初值多么小,当1<p<1+2/n(n≥3)时,解会在有限时间内破裂;且当1<p<1+2/n时,得到了解的生命跨度上界估计.证明过程中运用了试探函数法.【期刊名称】《丽水学院学报》【年(卷),期】2018(040)002【总页数】9页(P1-9)【关键词】半线性波动方程;破裂;外问题;耗散;狄利克雷边界条件【作者】徐根海;吴邦【作者单位】丽水学院工学院,浙江丽水323000;浙江理工大学理学院,浙江杭州310018【正文语种】中文【中图分类】O1750 引言考虑带狄利克边界条件的小初值耗散半线性波动方程外问题可以用公式表示为：其中=Rn\B1表示在 Rn（n≥3)上单位球 B1的补集，ε＞0 表示初值的小性。
初值（u0，u1）满足：对于在Rn中的Cauchy问题，已有的研究结果表明其中存在一个临界指标=1+，该指标称为Fujita 指标[1]，见 Nakao 和 Ono[2]、Li和 Zhou[3]、Todorova 和Yordanov[4]、Zhang[5]等学者的文献。
破裂情形的生命跨度研究见Nishihara[6]、Ikeda和Ogawa[7]及Lai和Zhou[8]等学者的文献。
外区域上小初值耗散半线性波动方程的初边值问题（1），也引起了很多人的关注，研究成果可见Ikehata[9-11]，Nakao[12]，Racke[13-14]，Shibata[15]，Ikehata[16-18]，Lai和 Yin[19]，Lin、Jiang 和 Yin[20]及 Wu、Ma 和 Jin[21]等学者的文献。
本文主要研究问题（1）解的有限时间破裂及生命跨度估计。
Ogawa等[22]证明了当1＜p＜1+时，带狄利克雷边界条件的初边值外问题解会在有限时间内破裂，但是并没有给出生命跨度估计。
2019年度广东省科学技术奖公示信息项目名称液体-气体两相流模型的适定性理论主要完成单位单位1：华南理工大学单位2：西北大学单位3：华中师范大学主要完成人（职称、完成单位、工作单位）1.朱长江（职称：教授、工作单位：华南理工大学、完成单位：华南理工大学、主要贡献：提出了完成本项目研究的主要思路，并实施了本项目所有创新点的主要科研工作。
10篇代表作全部都有他的署名并在研究中做出了决定性的贡献。
先后主持完成国家杰出青年科学基金项目和国家自然科学基金重点项目等项目。
）2.姚磊（职称：教授、工作单位：西北大学、完成单位：西北大学、主要贡献：参与本项目主要科研工作。
10篇代表作中有5篇都有他的署名并在研究中做出了重要贡献。
先后主持国家自然科学基金青年项目和面上项目。
基于本项目的主要研究工作，于2012年获全国百篇优秀博士学位论文奖）3.温焕尧（职称：教授、工作单位：华南理工大学、完成单位：华中师范大学、主要贡献：重要科学发现第二项的代表性论文2。
提出了梯度平方分解恒等式。
）项目简介刻画流体之间相互作用的液体-气体两相流模型是石油工业中描述管道和深井中油和气的生产和输运的常见数学模型。
该模型不仅具有深刻的物理意义，而且也具有重要的数学理论价值。
关于其研究是近二十多年来本领域的热点问题之一,有许多关于该模型及其相关模型的数值结果，但对于该模型的适定性理论，即存在性、唯一性和稳定性等结果却很少。
该项目系统地研究了液体-气体两相流模型的自由边界问题、初边值问题和Cauchy问题解的适定性等问题。
挪威应用数学家Steinar Evje 教授在其同一篇论文中提到了我们的其中1篇代表作11次并作为后续研究。
我们在研究爆破机制时提出了速度的梯度平方分解恒等式，从而代替了传统方法中的梯度平方分解不等式。
美国《数学评论》(MR3457694)对我们的一篇代表作进行了评论，认为是文章有趣且非常有技巧。
本项目第一完成人朱长江教授曾获国家杰出青年科学基金资助以及入选了万人计划“国家教学名师”，所领导的两相流模型研究团队被同行专家称为国内外该领域的两个团队之一“Zhu’s group” (具体请见[Lizhi Ruan, Proceedings of the Royal Society of Edinburgh, 144A,351-362, 2014]第352页)，另外一个团队为挪威应用数学家Steinar Evje教授团队。
Notes on Quasiminimality and ExcellenceJohn T.BaldwinDepartment of Mathematics,Statistics and Computer ScienceUniversity of Illinois at Chicago∗April7,2004AbstractThis paper ties together much of the model theory of the last50years.Shelah’s attempts to generalize the Morley theorem beyondfirst order logic led to the notion of excellence,which is a key to the structure theory of uncountable models.The notion of Abstract Elementary Class arose naturally in attempting toprove the categoricity theorem for Lω1,ω(Q).More recently,Zilber has attempted to identify canonicalmathematical structures as those whose theory(in an appropriate logic)is categorical in all powers.Zilber’strichotomy conjecture forfirst order categorical structures was refuted by Hrushovski,by the introducion of aspecial kind of Abstract Elementary Class.Zilber uses a powerful and essentailly infinitary variant on thesetechniques to investigate complex exponentiation.This not only demonstrates the relevance of Shelah’smodel theoretic investigations to mainstream mathematics but produces new results and conjectures inalgebraic geometry.Zilber proposes[63]to prove‘canonicity results for pseudo-analytic’rmally,‘canonical’means ‘the theory of the structure in a suitable possibly infinitary language(see Section2)has one model in each uncountable power’while‘pseudoanalytic’means‘the model of power2ℵ0can be taken as a reduct of an expansion of the complex numbers by analytic functions’.This program interacts with two other lines of research.First is the general study of categoricity theorems in infinitary languages.After initial results by Keisler,reported in[31],this line was taken up in a long series of works by Shelah.We place Zilber’s work in this context.The second direction stems from Hrushovski’s construction of a counterexample to Zilber’s conjecture that every strongly minimal set is‘trivial’,‘vector space-like’,or‘field-like’.This construction turns out to be a very concrete example of an Abstract Elementary Class,a concept that arose in Shelah’s analysis. And the construction is a crucial tool for Zilber’s investigations.This paper examines the intertwining of these three themes.For simplicity,we work in a countable vocabulary.The study of(C,+,·,exp)leads one immediately to some extension offirst order logic;the integers with all their arithmetic arefirst order definable in(C,+,·,exp).Thus,thefirst order theory of complex exponentiation is horribly complicated;it is certainly unstable and so itsfirst order theory cannot be categorical in power.That is,thefirst order theory of complex exponentiation cannot have exactly one model in each uncountable cardinal. One solution is to use infinitary logic to pin down the pathology.Insist that the kernel of the exponential map isfixed as a single copy of the integers while allowing the rest of the structure to grow.We describe in Section5 Zilber’s theorem that,modulo certain(very serious)algebraic hypotheses,(C,+,·,exp)can be axiomatized bya categorical Lω1,ω(Q)-sentence.The notion of amalgamation is fundamental to model theory.Even in thefirst order case,the notion is subtle because there are several variants depending on the choice of a class of models K and a notion≺of substructure.∗Partially supported by NSF grant DMS-0100594and CDRF grant KM2-2246.1The pair(K,≺)has the amalgamation property if whenever M∈K is embedded by f0,f1into N0,N1so that the image of the embeddings f0M,f1M≺N0,N1respectively,there is an N∗and embeddings g0,g1of N0,N1 into N∗with g0f0and g1f1agreeing on M.If K is the class of models of a completefirst order theory then the amalgamation property holds with≺as elementary embeddings of models.If K is the class of substructures of models of a complete quantifier eliminablefirst order theory then the amalgamation property holds for≺as arbitrary embeddings.Morley[39]observed that by adding names for each definable relation,we can assume, for studying the spectrum problm,that anyfirst order theory has elimination of quantifiers.Shelah[47],noted that this amalgamation hypothesis allows us to assume the existence of a‘monster model’which serves as a universal domain.In this domain the notion of type of an element a over a set A can be thought of either semantically as the orbit of a under automorphisms thatfix A or syntactially as the collection of formulas with parameters from A that are satisfied by a.Of course,the extension fromfirst order logic causes the failure of the compactness theorem.For example,it is easy to write a sentence in Lω1,ωwhose only model is the natural numbers with successor.But thereare some more subtle losses.The duality between the syntactic and semantic concept of type depends on the amalgamation property.Here is a simple example showing that amalgamation fails in models of a sentence ofLω1,ω.Consider the theory T of a dense linear order without endpoints,a unary predicate P(x)which is denseand codense,and an infinite set of constants arranged in order typeω+ω∗.Let K be class of all models of T which omit the type of a pair of points,which are both in the cut determined by the constants.Now consider the types p and q which are satisfied by a point in the cut,which is in P or in¬P respectively.Now p and q are each satisfiable in a member of K but they are not simultaneously satisfiable.So the amalgamation property has failed for K and elementary embeddings.This shows that a more subtle notion than consistency is needed to describe types in this wider context.We took‘canonical’above as meaning‘categorical in uncountable cardinalities’.That is,the class has exactly one model in each uncountable cardinality.The analysis offirst order theories categorical in power is based on first studying strongly minimal sets.A set is strongly minimal if every definable subset of it isfinite or cofinite.A natural generalization of this,particularly since it holds of simply defined subsets of(C,+,·,exp),is to consider structures where every definable set is countable or cocountable.As we will see,the useful formulation of this notion requires some auxiliary homogeneity conditions.The role of homogeneity in studying categoricity in infinitary languages has been known for a long time.There is a rough translation between‘homogeneity’hypotheses on a model and and corresponding‘amalgamation’hypotheses on the class of substructures of the model(Section2).A structure isℵ1-homogeneous if for any two countable sequences a,b,which realize the same type,and any c,there is a d such that a c and b d realize the same type.Thus,ℵ1-homogeneity corresponds to amalgamation over arbitrary countable subsets.Keisler[31]proved the natural generalization of Morley’stheorem for a sentenceψin Lω1,ωmodulo two assumptions:1.Every model ofψhas arbitrarily large elementary extensions.2.Every model ofψisℵ1-homogeneous.Keisler asked whether everyℵ1-categorical sentence in Lω1,ωsatisfies assumption2.The answer is no.Marcus[37]gave an example of a minimal prime model with infinitely many indiscernibles and a modification by Shelahprovides an example of a totally categorical(categorical in each uncountable cardinality)sentence in Lω1,ωwhich has noℵ1-homogeneous models.Shelah’s notion of an excellent class(extremely roughly:‘amalgamation over(independent)n-dimensional cubes for all n’and‘ℵ0-stability’)provides a middle ground.An excellent class(See paragraph2.0.9.)is a strengthening of Keisler’sfirst assumption(provides not only arbitrarily large models but a certain control over their construction)while weakening the second to assert amalgamation only over certain configurations.Recall that the logic L(Q)adds tofirst order logic the expression(Qx)φ(x)which holds if there are uncountably many solutions ofφ.I had asked whether a sentence in L(Q)could have have exactly one model and that model2have cardinalityℵ1.Shelah proved in[45]using that anℵ1-categorical sentence in Lω1,ω(Q)must have a modelof powerℵ2.There is a beautiful proof of this result in ZFC in[53].Shelah has moved this kind of argument from(ℵ1,ℵ2)to(λ,λ+)in a number of contexts.But,getting arbitrarily large models just from categoricity in a single cardinal has remained intractable,although Shelah reported substantial but not yet written progress in the summer of2003.Shelah proved an analogue to Morley’s theorem in[48,49]for‘excellent’classes defined in Lω1,ω.Assuming2ℵn<2ℵn+1,for all n<ω,he also proved the following kind of converse:every sentence in Lω1,ωthat iscategorical inℵn for all n<ωis excellent and categorical in all cardinals.The assumption of categoricityall the way up toℵωis shown to be essential in[18]by constructing for each n a sentenceψn of Lω1,ωwhichis categorical up toℵn but has the maximal number of models in all sufficiently large cardinalities.He alsoasserted that these results‘should be reproved’for Lω1,ω(Q).This‘reproving’has continued for20years andthefinale is supposed to appear in the forthcoming Shelah[50,51].Zilber’s approach to categoricity theorems is more analogous to the Baldwin-Lachlan approach than to Morley’s. Baldwin-Lachlan[8]provide a structural analysis;they show each model of anℵ1-categorical theory is prime over a strongly minimal set.This allows one to transfer the‘geometric’proof of categoricity in power for a strongly minimal theory to show categoricity inℵ1implies categoricity in all cardinalities.In fact,Zilber considers only the quasiminimal case.But a‘Baldwin-Lachlan’style proof was obtained by Lessmann for homogeneous model theory in[35]and for excellent classes in[34].That is,he proves every model is prime and minimal over a quasiminimal set.We begin in Section1by recalling the basic notions of the Fra¨ıss´e construction and the notion of homogeneity.In Section2,we sketch some results on the general theory of categoricity in non-elementary logics.In particular,we discuss both reductions to the‘first order logic with omitting types’and the‘syntax-free’approach of Abstract Elementary Classes.We turn to the development of the special case of quasiminimal theories in Section3. This culminates in Zilber’sfirst approximation of a quasiminimal axiomatization of complex exponentiation.In Section4we formulate the generalized Fra¨ıss´e construction and place it in the setting of Abstract Elementary Classes.We analyze this method for constructingfirst order categorical theories;we then see a variant to get examples in homogeneous model theory.Then we discuss the results and limitations of the program to obtain analytic representations of models obtained by this construction.Finally in Section5we return to Zilber’s use of these techniques to study complex exponentiation.We describe the major algebraic innovations of his approach and the innovations to the Hrushovski construction which result in structures that are excellent but definitely notfirst order axiomatizable.Many thanks to Rami Grossberg and Olivier Lessmann,who were invaluable in putting together this survey, but are not responsible for any ments by Assaf Hasson,David Kueker,David Marker,Charles Steinhorn,Saharon Shelah,and Boris Zilber improved both the accuracy and the exposition.We particularly thank the referee and editor for further clarifying the expositition.1The Fra¨ıss´e ConstructionIn the early1950’s Fra¨ıss´e[13]generalized Hausdorff’s back and forth argument for the uniqueness of the rationals as a countable dense linear order(without end points).He showed that any countable class K offinite relational structures closed under substructure and satisfying the joint embedding and amalgamation properties (see Definition4.1.6)has a unique countable(ultra)-homogeneous member(denoted G):any isomorphism betweenfinite subsets of G extends to an automorphism.There are easy variants of this notion for locally finite classes in a language with function symbols.The existence of such structures is proved by iterating the amalgamation property and taking unions of chains.(See[21]for a full account.)J´o nsson[28]extended the notion to arbitrary cardinals and Morley-Vaught[40]created an analogous notion for the class of models offirst3order theories with elementary embeddings as the morphisms.They characterized the homogeneous universal models in this situation as the saturated models.In general the existence of saturated models in powerκrequires thatκ=κ<κandκ>2|L|;alternatively,one may assume the theory is stable.In particular,κ-saturated models areκ-homogeneous.Morley proved every uncountable model of a theory categorical in an uncountable power is saturated.Abstract versions of the Fra¨ıss´e construction undergird the next section;concrete versions dominate the last two sections of the paper.2Syntax,Stability,AmalgamationThis section is devoted to investigations of categoricity for non-elementary classes.We barely touch the immense literature in this area;see[15].Rather we just describe some of the basic concepts and show how they arisefrom concrete questions of categoricity in Lω1,ωand Lω1,ω(Q).In particular,we show how different frameworksfor studying nonelementary classes arise and some relations among them.Any serious study of this topic begins with[30,31].In its strongest form Morley’s theorem asserts:Let T be afirst order theory having only infinite models.If T is categorical in some uncountable cardinal then T is complete and categorical in every uncountable cardinal.This strong form does not generalize to Lω1,ω;take the disjunction of a sentence which is categorical in allcardinalities with one that has models only up to,say, ing both the upward and downward L¨o wenheim-Skolem theorem, L os[36]proved that afirst order theory that is categorical in some cardinality is complete.Since the upwards L¨o wenheim-Skolem theorem fails for Lω1,ω,the completeness cannot be deduced for this logic.However,if the Lω1,ω-sentenceψis categorical inκ,then,applying the downwards L¨o wenheim-Skolem theorem,for every sentenceφeitherψ→φor all models ofφhave cardinality less thanκ.So ifφandψareκ-categoricalsentences with a common model of powerκthey are equivalent.We say a sentence of Lω1,ωis complete if iteither implies or contradicts every other Lω1,ω-sentence.Such a sentence is necessarilyℵ0-categorical(usingdownward L¨o wenheim-Skolem).Moreover,every countable structure is characterized by a complete sentence, which is called its Scott sentence.So if a model satisfies a complete sentence,it is L∞,ω-equivalent to a countablemodel.In particular,any model M ofψ∈Lω1,ωis small.That is,for every n it realizes only countably manyLω1,ω-n-types(over the empty set).Moreover,ifφhas a small model thenφis implied by a complete sentencesatisfied in that model.In thefirst order case it is trivial to reduce the study of categoricity to complete(for Lω,ω)theories.Moreover,first order theories share the fundamental properties of sentences–in particular,L¨o wenheim-Skolem down toℵ0.But an Lω1,ω-theory need not have a countable model.The difficulty is that an Lω1,ω-theory need not beequivalent to a countable conjunction of sentences,even in a countable language.So while we want to reducethe categoricity problem to that for complete Lω1,ω-sentences,we cannot make the reduction trivially.Wefirstshow that ifψ∈Lω1,ωhas arbitrarily large models and is uncountably categorical thenψextends to a completesentence.A key observation is that ifψhas arbitrarily large models thenψhas models that realize few types.Lemma2.0.1Supposeψ∈Lω1,ωhas arbitrarily large models.1.In every infinite cardinalityψhas a model that realizes only countably many Lω1,ω-types over the emptyset.2.Thus,if N is the unique model ofψin some cardinal,ψis implied by a consistent complete sentenceψwhich holds of N.Proof.Sinceψhas arbitrarily large models we can construct a model with indiscernibles(Chapters13-15of [31]).Now take an Ehrenfeucht-Mostowski model M forψover a set of indiscernibles ordered by a k-transitive4dense linear order.(A ordering is k-transitive if any two properly ordered k-tuples are in the same orbit under the automorphism group.These orders exist in every cardinal;take the order type of an orderedfield.)Then for every n,M has only countably many orbits of n-tuples and so realizes only countably many types in anylogic where truth is preserved by automorphism–in particular in Lω1,ω.Ifψisκ-categorical,letψ be theScott sentence of this Ehrenfeucht-Mostowski model with cardinalityκ. 2.0.1 If we do not assumeψhas arbitrarily large models the reduction to complete sentences,sketched below,is more convoluted and uses hypotheses(slightly)beyond ZFC.In particular,the complete sentenceψ does nothold,a priori of the categoricity model.The natural examples of Lω1,ω-sentences which have models of boundedcardinality(e.g.a linear order with a countable dense subset,or coding up an initial segment of the Vαhierarchy of all sets)have the maximal number of models in the largest cardinality where they have a model.Shelah discovers a dichotomy(Theorem2.0.2)between such sentences and‘excellent’sentences.We expand on the notion of excellence at2.0.9and later in the paper.For the moment just think of the assertion that a completeLω1,ω-sentence(equivalently,its class of models)is excellent as a step into paradise.For any class K of models,I(λ,K)denotes the number of isomorphism types of members of K,with cardinality λ.We may writeψinstead of K if K is the class of models ofψ.We say that a class K has many models of cardinalityℵn if I(ℵn,K)≥µ(n)(and few if not;there may not be any).We use as a black box the functionµ(n)(defined precisely in[49]).Either GCH or¬O#implyµ(n)=2ℵn but it is open whether it might be(consistently)smaller.The difficult heart of the argument is the following theorem of Shelah[48,49];we don’t discuss the proof of this result but just show how this solution for complete sentences gives the result forarbitrary sentences of Lω1,ω.Theorem2.0.2 1.(For n<ω,2ℵn<2ℵn+1)A complete Lω1,ω-sentence which has few models inℵn foreach n<ωis excellent(see2.0.9).2.(ZFC)An excellent class has models in every cardinality.3.(ZFC)Suppose thatφis an excellent Lω1,ω-sentence.Ifφis categorical in one uncountable cardinalκthen it is categorical in all uncountable cardinals.So a nonexcellent class defined by a complete Lω1,ω-sentenceψmay not have arbitrarily large models but,ifnot,it must have many models in some cardinal less thanℵω.Combining several results of Keisler,Shelah[48] shows:Lemma2.0.3Assume2ℵ0<2ℵ1.Letψbe a sentence of Lω1,ωthat has at least one but less than2ℵ1modelsof cardinalityℵ1.Thenψhas a small model of cardinalityℵ1.Proof.By Theorem45of[31],for any countable fragment L∗containingψand any N|=ψof cardinalityℵ1, N realizes only countably many L∗types over the empty set.Theorem2.2of[45]says that ifψhas a model M of cardinalityℵ1which realizes only countably many types in each fragment thenψhas a small model of cardinalityℵ1.We sketch a proof of that theorem.Add to the language a linear order<,interpreted as a linearorder of M with order typeωing that M realizes only countably many types in any fragment,write Lω1,ωas a continuous increasing chain of fragments Lαsuch that each type in Lαrealized in M is a formula in Lα+1. Add new2n+1-ary predicates and n+1-ary functions f n.Let M satisfy E n(α,a,b)if and only if a and b realize the same Lα-type and let f n map M n+1into the initialωelements of the order,so that E n(α,a,b) implies f n(α,a)=f n(α,b).Note:i)E n(β,y,z)refines E n(α,y,z)ifβ>α;ii)E n(0,a,b)implies a and b satisfy the same quantifier free formulas;iii)ifβ>α,E n(β,a,b)implies(∀x)(∃y)E n+1(α,x a,y b).Thus,iv) for any a∈M each equivalence relation E n(a,y,z)has only countably many classes.All these assertions canbe expressed by an Lω1,ωsentenceφ.Now add a unary predicate symbol P and a sentenceχwhich assertsthat M is an end extension of P(M).For everyα<ω1there is a model Mαofφ∧ψ∧χwith order type of5(P(M),<)greater thanα.(Start with P asαand alternately take an elementary submodel for the smallest fragment L∗containingφ∧ψ∧χand close down under<.Afterωsteps we have the P for Mα.)Now by Theorem12of[31]there is countable structure(N0,P(N0))such that P(N0)contains a copy of(Q,<)and N0 is an end extension of P(N0).By Theorem28of[31],N0has an L∗elementary extension of cardinalityℵ1.Fix an infinite decreasing sequence d0>d1>...in N0.For each n,define E+n(x,y)if for some i,E n(d i,x,y).Now using i),ii)and iii)prove by induction on the quantifier rank ofφthat N1|=E+n(a,b)implies N1|=φ(a)if andonly if N1|=φ(b)for every Lω1,ω-formulaφ.For each n,E n(d0,x,y)refines E+n(x,y)and by iv)E n(d0,x,y)has only countably many classes;so N is small. 2.0.3Using these two results,we easily derive a version of Morley’s theorem for an Lω1,ω-sentence.Theorem2.0.4Assume2ℵn<2ℵn+1for n<ω.If an Lω1,ω-sentenceψhas an uncountable model,then either1.ψhas many models inℵn for some n<ωor2.ψhas arbitrarily large models and ifψis categorical in one uncountable cardinalκthen it is categoricalall uncountable cardinals.Proof.Supposeψhas few models inℵn for each n<ω.By Lemma2.0.3,choose a small model ofψ,say with Scott sentenceψ .Assuming2ℵn<2ℵn+1for each n,Theorem2.0.21)impliesψ is excellent.By Theorem2.0.2 2)ψ and thusψhave arbitrarily large models.Now supposeψis categorical inκ>ℵ0.Then so isψ whence, by Theorem2.0.23),ψ is categorical in all uncountable powers.To showψis categorical aboveκnote that by downward L¨o wenheim-Skolem all models ofψwith cardinality at leastκsatisfyψ ;the result follows by the categoricity ofψ .Ifψis not categorical in some cardinalityµ<κ, there must be a sentenceθwhich is inconsistent withψ but consistent withψ.Applying the entire analysis to ψ∧θ,wefind a complete sentenceψ which has arbitrarily large models,is consistent withψand contradicts ψ .But this is forbidden by categoricity inκ. 2.0.4 One corollary of this result isCorollary2.0.5Assume2ℵ0<2ℵ1.If an Lω1,ω-sentence is categorical inℵn for n<ω,then it is categoricalin all cardinalities.Hart and Shelah[18]have shown the necessity of the hypothesis of categoricity up toℵω.A key tool in the study of complete Lω1,ω-sentences is the reduction of the class of models of such sentences toclasses which are‘closer’to beingfirst order.We now give a full account of this easy reduction.Chang proved in[12]that the class of models of any sentence in Lκ+,ωcould be viewed as the class of reducts to L of models of afirst order theory in an expansion L of L which omitted a family of types.Chang(Lopez-Escobar[12])used this observation to prove that the Hanf number for Lκ+,ωis same as the Hanf number for omitting a family ofκtypes.Shelah[45]took this reduction a step further and showed that the class of models of a complete sentencein Lω1,ωare in1-1correspondence(mapping L∞,ω-submodel to elementary submodel)with the class of atomicmodels of an appropriatefirst order theory in an expanded language.That is,to study the generalization ofMorley’s theorem to complete Lω1,ω-sentences it suffices to study classes of structures defined by a special typeoffinite diagram.By afinite diagram we mean an EC(T,Γ)class:those models offirst order theory that omit all types from a specified collectionΓof types infinitely many variables over the empty set.Abusing the EC(T,Γ)notation,EC(T,Atomic)denotes the class of atomic models of T(i.e.to conform to the notationwe should write nonatomic).Most detailed study of the spectrum of Lω1,ω-sentences[45,48,49,34,16,27]just work withfinite diagrams or more restrictively atomic models(and usually under stronger homogeneity conditions).In general,an atomic class might be defined by omitting uncountably many types;in the case of interest only countably many types have to be omitted.6Theorem2.0.6Letψbe a complete sentence in Lω1,ω.Then there is a countable language L extending Land afirst order L -theory T such that the reduct map is1-1from the atomic models of T onto the models ofψ.Proof.Let L∗be a countable fragment of Lω1,ωwhich contains all subformulas ofψand the conjunction ofeach Lω1,ω-type that is realized in a model ofψ.(This set is countable since complete sentences are small.)Expand L to L by inductively adding a predicate Pφ(x)for each L∗-formulaφ.Fix a model ofψand expand it to an L -structure by interpreting the new predicates so that the new predicates represent eachfinite Boolean connective and quantification faithfully:E.g.P¬φ(x)↔¬Pφ(x),andP(∀x)φ(x)↔(∀x)Pφ(x),and that,as far asfirst order logic can,the Pφpreserve the infinitary operations:for each i,P Vi φi(x)→Pφi(x).Let T be thefirst order theory of any such model and consider the countable setΓof typesp Vi φi(x)={¬P Viφi(x)}∪{Pφi(x):i<ω}.Note that if q is an Lω1,ω-type realized in a model of T,P V q generates a principal type in T.Now if M is amodel of T which omits all the types inΓ(in particular,if M is an atomic model of T),M|L|=ψand each model ofψhas a unique expansion to a model of T which omits the types inΓ(since this is an expansion bydefinitions in Lω1,ω). 2.0.6So in particular,any complete sentence of Lω1,ωcan be replaced(for spectrum purposes)by considering theatomic models of afirst order theory.Since all the new predicates are Lω1,ω-definable this is the naturalextension of Morley’s procedure of replacing eachfirst order formulaφby a predicate symbol Pφ.Morley’s procedure resulted in a theory with elimination of quantifiers thus guaranteeing amalgamation over sets for first order categorical T.A similar amalgamation result does not follow in this case.Nor,In general,dofinite diagrams satisfy the upwards L¨o wenheim-Skolem theorem.Remark2.0.7(Lω1,ω(Q))The situation for Lω1,ω(Q)is more complicated.The example[18]of a sentenceof Lω1,ωthat isℵ1-categorical and not categorial in all uncountable powers is quite complicated.But theL(Q)theory of two disjoint infinite sets illustrates this phenomena trivially.Some of the analysis of[48,49] goes over directly.But many problems intervene and Shelah has devoted several articles(notably[52,50,51]tocompleting the analysis;a definitive version has not appeared.The difficulty in extending from Lω1,ωto Lω1,ω(Q)is in constructing models with the proper interpretation of the Q-quantifier.Following Keisler’s analysis of this problem in[30]the technique is to consider various notions of strong submodel.Two notions are relevant:in the first,the relation of M≺K N holds when definable sets which are intended to be countable(M|=¬(Qx)φ(x)) do not increase from M to N.The seconds adds that definable sets intended to be uncountable(M|=(Qx)φ(x)) increase from M to N.Thefirst notion gives an AEC(Definition2.0.8);the second does not.The reduction [53,50]is actually to an AEC along with the second relation as an auxiliary that guarantees the existence of standard models.When J´o nsson generalized the Fra¨ısse construction to uncountable cardinalities[28,29],he did so by describ-ing a collection of axioms,which might be satisfied by a class of models,that guaranteed the existence of a7。
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第一部分（共20小题，每小题1.5分，满分30分）1．She did not feel a bit nervous though it was the first time she ___________ in public.A．spoke B．have spokenC．had spoken D．were speaking2．A scientist’s attempt to produce the world’s first gene-edited babies _____ are immune to HIV has sparked controversy in academia and the public.A．as B．whoC．whom D．whose3．— Where is Gary?— He is at the library studying for his German test on Wednesday. In fact, he ________ for the test every day over the past week.A．has reviewed B．was reviewingC．has been reviewing D．had been reviewing4．________ the concert to raise money for hunger relief and to make the public aware of the problem, Geldof invited many famous musicians to take part in it.A．Intended B．Intending C．Having intended D．To intend5．Lifting off at sunrise, the hot air balloon goes wherever the wind blow.A．may B．must C．need D．should6．All the photographs in this book, stated otherwise, date from the 1950s.A．unless B．until C．once D．if7．After nine years working to protect Siberian tigers, Yang Jun _____ his efforts recognized at the annual award ceremony in Beijing where he was named a "wildlife protector".A．had B．had hadC．has had D．has8．Christine is ______ about anything but her dress as she doesn’t really care about what she looks like in the eyes of others.A．cautious B．optimisticC．particular D．curious9．Many people may be surprised by the number of chemicals that they are ________ to through everyday householdproducts.A．devoted B．addicted C．exposed D．applied10．That’s far more than the number that have actually ______ in recent years.A．come to light B．come to life C．come across11．As a surgeon,I cannot any mistakes;it would be dangerous for the patient.A．appreciate B．removeC．offer D．afford12．_____what they say about me, I’m go ing to continue my work.A．In terms of B．Regardless ofC．Instead of D．In favor of13．Encourage your children to try new things, but try not to _________________them too hard.A．draw B．strikeC．rush D．push14．Water Knows Answers by a Japanese author, ________ into 16 languages, has become a best- seller all over the world.A．having translated B．to be translatedC．being translated D．translated15．Think carefully before you answer questions online. Y ou may be ______ into giving away very important personal information.A．caught B．addictedC．seized D．trapped16．We have strong ________ for believing that the newly-invented material is not only popular but also a monument to the eco-friendly lifestyle.A．grounds B．reasons C．causes D．purposes17．It’s _______ for pe ople to blame traffic jams, the cost of gas and the great speed of modern life.A．reasonable B．availableC．accurate D．cautious18．The English in this story has been simplified to make it easier _______.A．to be understood B．understood C．understanding D．to understand19．_______ that I wouldn’t support myself at that moment.A．I was weak enough B．I was too weakC．So weak was I D．Such weak was I20．One hundred and fifteen Chinese companies hit the newly released Fortune 500 list for 2017, ______ an increase for the 14th straight year, domestic website reported on Thursday.A．to realize B．realizedC．realizing D．having realized第二部分阅读理解（满分40分）阅读下列短文，从每题所给的A、B、C、D四个选项中，选出最佳选项。
Local Search Algorithmsfor aSingle-Machine Scheduling ProblemwithPositive and Negative Time-LagsJohann HurinkUniversity of Twente,Faculty of Mathematical SciencesP.O.Box217,NL–7500AE Enschedee-mail:j.l.hurink@math.utwente.nlJens KeuchelUniversit¨a t Osnabr¨u ck,Fachbereich Mathematik/InformatikD–49069Osnabr¨u cke-mail:gjkeuche@informatik.uni-osnabrueck.deMay1998AbstractPositive and negative time-lags are general timing restrictions between the starting times of jobs which have been introduced in connection with the Metra Potential Method.Although very powerful,these relations have been considered only seldom in the literature since already for a single machine problem with positive and negative time-lags the problem offinding a feasible solution is N P-complete.In this paper a local search approach for a single-machine scheduling prob-lem with positive and negative time-lags and the objective to minimize the makespan is presented.Since the existence of a feasible initial solution for starting the search cannot be guaranteed,infeasible solutions are incorpo-rated into the search putational results based on instances resulting from shop problems are reported.Key words:time-lags,tabu search,schedulingSubject Classification:90B351IntroductionAlmost40years ago,Roy[1959]introduced a simple but powerful tool tomodel project networks.For his Metra Potential Method(MPM)he used two types of relations between activities.Thefirst type expresses that between the starting points of two activities there must be a minimal(positive)time-lag.The second stipulates that there must be a maximal(negative)time-lag. Although these time-lags were extremely useful for modeling projects,they were hardly used over the ensuing three decades.Results of Bartusch et al.[1988]and Brucker et al.[1996]may explain this atfirst sight surprising fact. Whereas Bartusch et al.[1988]show that arbitrary time-lags may be usedto model release times and deadlines of activities and time-dependent re-source availability,Brucker et al.[1996]show that many complex scheduling problems like general shop problems,problems with multi-processor tasks,problems with multi-purpose machines,and problems with changeover times can be reduced to a single-machine problem with arbitrary time-lags.As a consequence,already for very simple resource-constrained scheduling prob-lems with arbitrary time-lags between the activities,the question whether or not a feasible schedule exists is N P-complete.In recent years,arbitrary time-lags were reanimated again.Several papers developing heuristics or branch-and-bound methods occurred mainly in thearea of project scheduling(Bartusch et al.[1988],Brinkmann and Neumann [1996],De Reyck and Herroelen[1996],Elmaghraby and Kamburowski[1992], Neumann and Schwindt[1995],Zhan[1994]).Besides this,some researchwas carried out for single-machine problems.Balas et al.[1995]propose an efficient adaption of Carlier’s algorithm for the single-machine problemwith heads and tails1|r i,q i|C max(Carlier[1982])to the case with positive time-lags.Wikum et al.[1994]investigated the complexity of single-machine problems with minimal and/or maximal distances between jobs.Their mainresults show that scheduling problems of this type with a very simple struc-ture are already N P-hard.Furthermore,Brucker et al.[1996]provide a branch-and-bound approach for solving a single-machine problem with arbi-trary time-lags.In this paper we will consider the application of local search to the single-machine problem with positive and negative time-lags and the objective to minimize the makespan.Based on the reductions presented by Brucker et al.[1996]such heuristics may be seen as a type of general purpose heuris-tics since in principle they can also be used to solve various other types of scheduling problems.A main obstacle for developing local search heuristics for the considered problem is the fact that the determination of a starting1solution is already a hard problem.We will overcome this by incorporating infeasible solutions in the search process.More precisely,we will consider all sequences of jobs which are admissible for the positive time-lags only as solutions regardless of whether a feasible schedule respecting this sequence exists or not.The paper is organized as follows.In Section2we will give a formal defini-tion of the considered problem and present a network representation for the instances.Afterwards,in Section3we will formulate the problem offinding the best schedule for a given sequence of the jobs(if such a schedule exists) as a longest path problem and develop an efficient method to determine such a schedule.Section4will be devoted to the description of local search meth-ods.A basic method will be a tabu search approach which starts with an ’infeasible’sequence of the jobs and which tries to guide the search to a fea-sible sequence.This basic routine will be packed in an outer method which has two features.Whenever a feasible solution has been detected through the basic routine,the given instance will be changed in such a way that the current solution becomes infeasible and only improving solutions of the cur-rent solution correspond to feasible solutions of the changed instance.After the change of the instance the basic routine starts again with the current so-lution which is now infeasible and tries tofind a feasible and thus improving solution.If after a certain amount of time the basic routine is not able to find a feasible schedule,the outer method has a second task.It applies di-versification operators to the current solution in order to reach another area of the search space which perhaps contains a feasible solution.In Section 5computational results using instances which are generated by reductions of benchmark instances for job-shop and open-shop problems are presented. The paper ends with some concluding remarks.2The ProblemIn this section we will give a formal definition of the considered problem and introduce some useful notations.Let J={1,...,n}be a set of n jobs with processing times p1,...,p n to be scheduled without preemption on a single machine.Furthermore,we have a set of relations called time-lags of the formS i+d ij≤S j for all(i,j)∈R,(2.1) where R⊆J×J and S i denotes the starting time of job i,i=1,...,n (all data are assumed to be integral).If d ij≥0,then(2.1)means that2job j cannot be started earlier than d ij time units after the starting time of job i(minimal time-lag).On the other hand,if d ij<0,then job j cannot be started earlier than|d ij|time units before the starting time of job i (maximal time-lag).Since we consider non-preemptive solutions,a schedule of the jobs is uniquely defined by a vector of starting times S1,...,S n for the jobs.Such a schedule S=(S1,...,S n)is called a feasible schedule if•for all i=j the intervals[S i,S i+p i[and[S j,S j+p j[are disjoint and •the time-lags(2.1)are satisfied.The goal is tofind a feasible schedule which minimizes the makespan C max= maxi=1,...,n{S i+p i}if such a schedule exists.As mentioned in the introduction, for this problem the question of whether or not a feasible solution exists,is already N P-complete.For convenience of notation,we will add two dummy jobs0and n+1with processing times zero to the job set J.Job0is a starting job which must be“processed”before all other jobs i(i.e.d0i=0)and job n+1is a final job which must be“processed”after all other jobs i(i.e.d i,n+1=p i). Furthermore,we may assume that for each job-pair(i,j)∈(J∪{0,n+ 1})2,i=j,a relation S i+d ij≤S j is defined(if this is not the case,we introduce a redundant relation with d ij=−∞).To represent the time-lags R,we will make use of a network N(R)which is defined as follows:•the vertex set V={0,1,...,n,n+1}consists of all jobs1,...,n and the two dummy jobs0and n+1;•for each pair(i,j)of jobs i,j=0,...,n+1with i=j there is an arc (i,j)of length d ij.If we calculate in this network N(R)the longest path lengths l(i)from the starting job0to all jobs i,i=1,...,n+1(if this is not possible since N(R) contains a positive cycle,the given instance has no feasible schedule),the schedule where job i starts at time l(i),i=1,...,n,respects the time-lags, but is not necessarily feasible since jobs may overlap.Before discussing methods of how to solve these conflicts of overlapping jobs, we would like to mention two techniques to sharpen the given time-lags(for3details and further methods,see Bartusch et al.[1988],Brucker et al.[1996]).Firstly,we may replace the length d ij of a given time-lag by the length of a longest path from i to j in N(R).Furthermore,if d ij∈]–p j,p i[,we may replace d ij by p i since job j cannot be scheduled before job i.In thefollowing we will assume that the given data of the problem has been modified according to these two rules.3Fixed Job SequencesIn the previous section we have noticed that a main scheduling decision for the considered problem is to determine an order in which the jobs will be scheduled.However,in contrast to most other non-preemptive one-machine scheduling problems,afixed decision for all pairs of jobs(i.e.afixed se-quence for the jobs)does not directly lead to a corresponding schedule,since due to negative time-lags(maximal distances)we cannot apply simple list scheduling heuristics.In the remaining part of this section we will present an efficient method to determine a best schedule of the jobs which respects the given time-lags and afixed sequence of the jobs if such a schedule exists. Letπbe an arbitrary sequence of the jobs.In each schedule(S i)n i=1respecting the sequenceπwe must haveSπ(i)+pπ(i)≤Sπ(i+1)for all i∈{1,...,n−1},(3.1) i.e.between the jobsπ(i)andπ(i+1)we have a time-lag of length pπ(i). Thus,each feasible schedule respectingπhas to respect the time-lags dπij defined bydπij= max{p i,d ij}if i=π(k),j=π(k+1),d ij otherwise.By N(Rπ)we will denote the network N(R)where the arc lengths are given by the values dπij.To calculate the best schedule respectingπwe may calculate in the network N(Rπ)the lengths lπ(i)of the longest paths from the starting job0to all jobs i,i=1,...,n+1.If the network contains a positive cycle,clearly no feasible schedule respectingπexists.Otherwise,the schedule where each job i starts at time lπ(i)is feasible(due to(3.1)no jobs overlap)and has a makespan of length Cπmax=lπ(n+1)which is minimal over all schedules respectingπ.4In principle,the values lπ(1),...,lπ(n+1)can be calculated using the Floyd-Warshall algorithm(see,e.g.Ahuja et al.[1993]).The complexity of this al-gorithm is O(n3)and in the case where no positive cycle exists,this algorithm really executes n3steps.Since our goal is to carry out local search on the set of all possible job sequences,the calculation of the values lπ(1),...,lπ(n+1) will be executed very often which will mainly determine the computational time for the local search approaches.Therefore,in the following we will present an alternative longest path method which takes advantage of the structure of the time-lags dπij.The resulting method will also have a worst time complexity of O(n3).However,in the worst case approximately at the most1It remains to show that the algorithm CalculateSchedule(π)terminates and to estimate its running time.Lemma1If the network N(Rπ)does not contain a cycle of positive length, the algorithm CalculateSchedule(π)terminates after at the most n−1least one arc which represents a real minimal time-lag(i.e.dπij>p i)and at least one arc with length dπij=p i must occur.The second part of the statement follows from the fact that the time-lags given by the instance are assumed to be transitive closed.Thus,by Claim2the path P k has at themost⌈n−22arcs of negative length.Claim1now proves the lemma.2To estimate the running time of the algorithm CalculateSchedule(π)it re-mains to estimate the complexity of the procedure improve(S,π).Lemma2If the network N(Rπ)does not contain a cycle of positive length, in the l-th iteration of the WHILE-loop of the algorithm CalculateSchedule(π) at the most n−2l values S i can be changed by the procedure improve(S,π). Proof:If in the l-th iteration one value S i is changed,the network N(Rπ) contains at least one longest path P k from0to a vertex k with at least l arcs of negative length.As mentioned in the proof of Lemma1,each path P k starts with at least two arcs of non-negative length and contains no two consecutive arcs with negative lengths.Thus,before the l-th arc of negative length on P k at least2l vertices(excluding0)occur.The longest paths to these vertices contain less than l arcs of negative length and,thus,the corresponding S i-values are alreadyfixed before iteration l(see Claim1in the proof of Lemma1).2 Using these two lemmata we get:Theorem3Using the results of Lemma1and2as stopping conditions,the algorithm CalculateSchedule(π)executes at the most approximately12the current schedule is not feasible.In this case the networkN(Rπ)contains a positive cycle and the sequenceπdoes not represent a feasible solution.The calculation of the initial solution needs O(n2)steps.By combining Lemmata1and2we can bound the number of changes of S i-values byn−12)2.If directly after each change of an S i-value,we calculate the influence of the7new S i-values together with the time-lags dπij on the S j-values(n−1steps), we get the stated upper bound on the number of steps.2 If the algorithm terminates with a feasible schedule S,it will be of impor-tance for the definition of suitable neighborhoods(see Section4)to identify the longest paths corresponding to the starting times of the jobs.To be able to reconstruct these paths,a vector pre will be introduced,where pre(i)will denote the predecessor of i on the longest path P i determining the current value of S i.After an initialization depending on the initial partially fea-sible schedule,these values always have to be updated if in the procedure improve(S,π)a value S i is increased.It remains to consider the case in further detail,where no feasible schedule w.r.t.πexists.The algorithm CalculateSchedule(π)is able to detect infea-sibility at two places:If after⌈n−2than our method.4A Local Search ApproachIn this section we will present a local search method for a single-machine problem with positive and negative time-lags.Firstly,we will discuss how we choose the search space and how we define the objective values of the solutions.Subsequently,we will introduce the neighborhoods which form the basis of the local search approach(for an introduction to the basic ideas of local search and neighborhoods,see e.g.Papadimitriou and Steiglitz[1982] or Aarts and Lenstra[1997]).Finally,we will describe the main structure of the developed local search heuristic.Since for the single-machine problem with positive and negative time-lags the search for a feasible solution is already N P-hard,we somehow have to deal with infeasible solutions.In our approach we will consider the set of all sequences of the jobs which are compatible with the non-negative time-lags as search space S,i.e.S={π=(π1,...,πn)|if dπiπj≥0then i<j}.In general,this set S will also contain sequences for which no corresponding feasible schedule exists.This causes problems for defining suitable objective values for the sequences.Whereby for’feasible’sequences we may define the objective value as the makespan of the best schedule w.r.t.π(calculated by the algorithm CalculateSchedule(π)),’infeasible’sequences are harder to tackle.For such sequencesπwe somehow have to measure the degree of infeasibility.To define such a measure,we will make use of the starting times of a partial feasible schedule S w.r.t.π.More precisely,we will apply the algorithm CalculateSchedule(π)and use the partial feasible schedule S with which we started the iteration in which the infeasibility was detected. The objective value ofπwill now be defined by a measure on the infeasibility of this partial feasible schedule S.Possible measures are:•Z1:number of violated time-lags in S,•Z2:value of the maximal violation of a time-lag in S,•Z3:sum of the violations of time-lags in S.At this point,it is important to note that these measures are not’sharp’, since they crucially depend on the time at which we stop the algorithm Calcu-lateSchedule(π).It may occur that the partial feasible schedule,which we9would achieve after one additional iteration,has a totally different measure of infeasibility.Although we have the possibility of calculating the objective values for feasi-ble and infeasible sequences with the same routine,it is rather inconvenient to have two different types of measures.To avoid this,we will artificially make all considered sequences infeasible.Whenever a feasible solution is de-tected,we will modify the given instance by increasing the time-lag d n+1,0 to the value−(K−1),where K is the makespan of the feasible solution. By bounding the distance between the starting job0and thefinal job n+1 by K−1,only schedules with a makespan smaller than K remain feasible. Thus,the current schedule becomes infeasible and the search for a feasible solution corresponds to the search for a solution which improves the best solution found so far.Based on the above-mentioned technique to always modify the given instance when a feasible schedule has been detected,we may define the neighborhoods for the local search method on the base of infeasible sequences.Since the neighborhoods mainly determine navigation through the search space,their definition should be based on the goal tofind feasible sequences.In the following theorem we will prove that this goal can only be achieved by specific changes to the given infeasible solution.To state the theorem wefirst have to introduce a special decomposition of cycles.Let C=(i1,...,i k,i k+1=i1)be a cycle in a network N(Rπ)(w.l.o.g.we will assume dπik i1=d ik i1).A subsequence B=(i j,...,i l)of C is calleda block if the lengths of the arcs within B are determined by the sequence π,and if the lengths of the two arcs which combine B with the rest of thecycle are determined by original time-lags,i.e.dπir i r+1=p ir>d ir i r+1,r=j,...,l−1,dπij−1i j =d ij−1i j,and dπil i l+1=d il i l+1.It is straightforward to seethat each cycle C decomposes uniquely into a sequence of blocks.Based onthis decomposition of cycles into blocks we can proveTheorem4Letπbe a given infeasible sequence of the jobs,let C=(i1,...,i k,i k+1= i1)be a cycle in N(Rπ)with positive length,and let B1,...,B r be the blocksof C.If,for some sequenceπ′,a corresponding feasible schedule exists,one of the following two properties holds:•inπ′at least one job of a block B i different to thefirst job in B i issequenced before all other jobs in B i or•inπ′at least one job of a block B i different to the last job in B i is10sequenced after all other jobs in B i .Proof :Let f (B i )(l (B i ))be the first (last)job of block B i ,i =1,...,r .Assume that for a feasible sequence π′none of the stated properties holds.Thus,all jobs of B i are sequenced between f (B i )and l (B i )in the sequence π′.This implies that the length of a longest path from f (B i )to l (B i )in N (R π′)is at least i ∈B i \{l (B i )}p i .Since,furthermore,we have d π′l (B i )f (B i +1)≥d πl (B i )f (B i +1),i =1,...,r (f (B r +1)=f (B 1)),the length of C in N (R π′)is greater than or equal to the length of C in N (R π)and,thus,positive.This implies that no feasible schedule w.r.t.π′exists,which is a contradiction.2Properties similar to those stated in the above theorem have been developed for shop problems and are used as a basis for branch-and-bound methods (see,e.g.Brucker et al.[1994],[1997])as well as for local search heuristics (see,e.g.Dell’Amico and Trubian [1993],Hurink et al.[1994],Nowicki and Smutnicki [1996]).The basis of these results is given in a work by Grabowski et al.[1986],in which the notation of a block is introduced in connection with a one-machine scheduling problem.In principle,the above theorem describes possible ways in which a cycle of positive length can be destroyed.Since the destruction of such cycles is necessary to obtain feasible solutions,we may use Theorem 4to define neighborhoods.Let πbe a given infeasible sequence and let C be a cycle of positive length in N (R π).Furthermore,let B 1,...,B r be the blocks ofC .The neighborhood N (π)of the sequence πis defined as the set of all sequences which are obtained by one of the following operators:•A job k ∈B i \{l (B i )}is moved directly after job l (B i ).•A job k ∈B i \{f (B i )}is moved directly in front of job f (B i ).Obviously,the number of sequences in a neighborhood N (π)depends on the number of jobs in the cycle C and the partition of this cycle into blocks.For the defined neighborhood we can prove that it is possible to reach a globally optimal solution independently of the initial solution.Theorem 5The given neighborhood N is weakly connected,i.e.for each solution πa sequence of solutions π1,...,πk exists such that π1=π,πi +1∈N (πi ),and πk is a globally optimal solution.Proof :W.l.o.g.we may assume that initially the length of the arc (n +1,0)is given by −K ,where K is the makespan of an optimal solution.Thus,we11only have to prove that it is possible to reach a feasible solution starting from an arbitrary solutionπ.Letπ′be a feasible solution.Due to Theorem4there exists a job j in some block B i of a positive cycle in N(Rπ)which is sequenced inπ′before or after all other jobs of block B i.If we move to the corresponding neighbor(move j before or after B i)we get a solution where j is sequenced relative to all other jobs of B i in the same way as inπ.If we iteratively repeat this process,we never will destroy the relative order between j and the other jobs of B i since we always choose the moving job according to Theorem4.Thus,after afinite number of steps we will get to a feasible solution.2 The definition of the neighborhood of a sequenceπdepends not only onπbut also on the chosen positive cycle C in N(Rπ).The connectivity of the defined neighborhood is independent of this choice.However,for a practical application we have to make a deterministic choice:If within the procedure CalculateSchedule(π)infeasibility is detected,we reconstruct the positive cy-cle which corresponds to the last considered time-lag.Otherwise,if Calcu-lateSchedule(π)produces a feasible schedule with makespan K,a critical path corresponding to this schedule together with the arc(n+1,0)with new length−(K−1)results in a cycle with length1.After defining the search space,the objective values,and the neighborhood, we can now describe the main structure of the developed local search ap-proach.The basis of this approach is a tabu search method which will be embedded in an outer method where some diversification of the search will be realized.The reason for applying diversification is that we do not want to give up the search at thefirst point at which the tabu search method gets stuck.Technically,this will be realized as follows:Whenever the tabu search method stops,we apply some diversification operator to the current best so-lution and again start tabu search with the changed solution.If this new run leads to an improvement,we repeat this process.Otherwise,if the last run of tabu search did not lead to an improvement,we apply some other diver-sification operator to the current best solution.We stop when all possible diversification operators have been applied without success.The structure of the outer method can be summarized as follows:12Algorithm Local Searchcalculate an initial sequenceπ;π′:=π;π:=TabuSearch(π);WHILE diversification is possible DOBEGINπ′:=diversification(π);π′:=TabuSearch(π′);IFπ′is better thanπTHENπ:=π′;END;In the above description the statement diversification is possible means that not all possible diversification operators have already been applied to the current best solutionπ.In our application of this algorithm we have consid-ered two possible diversification operators.They are dependent on the blocks B1,...,B r of the chosen positive cycle C in the network N(Rπ).More pre-cisely,thefirst diversification operator div1changes all blocks and the second diversification operator div2only changes every second block of C in the fol-lowing way:if|B i|≥4:exchange f(B i)with its successor andl(B i)with its predecessor,if|B i|≤3:exchange f(B i)and l(B i)if this is allowed.The initial sequenceπfor the local search approach has been calculated by a priority rule-based heuristic.In the i-th step of this heuristic a job will be sequenced at position i,i.e.jobπi will be determined,and a starting timeSπi for this job will befixed.Jobπi is chosen by a priority rule from theset of jobs which have predecessors w.r.t.the minimal time-lags only in the set{π1,...,πi−1}of already sequenced jobs.The starting time of jobπi will be determined such that jobπi is scheduled after jobπi−1and respects all minimal time-lags from jobs in{π1,...,πi−1}.Thus,the resulting schedule S is partially feasible w.r.t.π.As priority rules we have used:P1:choose the job with minimal earliest possible starting timeP2:choose the job with minimal latest possible starting timeP3:choose the job with maximal number of successors withrespect to the minimal time-lags only.It remains to describe the elements of the tabu search method we used in more detail.13•Tabu list managementWe have chosen a tabu list offixed length.This length will be defined√depending on the size of the instance(e.g.const·of the time-lags to job i(from job i)by one of the measures Z1,Z2,Z3 defined at the beginning of this section.•Calculation of the best non-tabu neighborIn a standard tabu search approach the best non-tabu neighbor is cho-sen as the next solution.However,as already mentioned,the objective measures for the infeasible sequences are not’sharp’.To incorporate this into our search process,we have not determined the next sequence as the best non-tabu neighbor w.r.t.one of the objective measures but as follows:By a primary measure Z we calculate a subset S1of good neighbors.More precisely,S1is the subset of all neighbored sequences for which the objective measure is within a distance∆(∆is a given constant)of the objective measure of the best neighbor.From this sub-set S1we choose as the next solution the best w.r.t.a second objective measure Z′.To reduce the computational times for one iteration of tabu search, we have also tested a’first-fit’strategy to determine the next solution.If,during the evaluation of the neighborhood of the current solution,a solution is detected which improves the current solution,we stopthe evaluation of the neighborhood and directly move to the improv-ing solution.Again,we will consider the unsharpness of the objective measures by only declaring a solution as an improving solution if its measure is at least∆units better than that of the current solution.If the neighborhood contains no improving solution(in the sense just de-fined),we use the selection mechanism described above.This strategy may reduce the computational times for one iteration at the price of a potential decrease in the quality of the next solution.•Stopping conditionWe will stop the tabu search method if in the last maxiter iterations the current best solution has not been improved,where maxiter is a given constant.5Computational ResultsIn the previous section we have described a local search approach for the single-machine problem with positive and negative time-lags.We imple-mented this method using the programming language C and we tested the algorithms on a SUN ULTRA,167MHz.15。
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生物多样性与生态系统功能之间的关系假说针对生物多样性与生态系统功能之间关系的复杂性，生态学家提出了许多假说来解释这些现象。
这些假说总的来说可以分成三个方面：第一类强调物种对生态系统功能均具有重要的作用，包括铆钉假说（rivet hypothesis）和单调/驼峰模型假说（monotonic/unimodal hypothesis）。
Ehrlich（1981 Ehrlich P R, Ehrlich A H. Extionciton: The Causes and Consequences of the Disappearance ofSpecies. Random House, New York, 1981.）提出了铆钉假说（rivethypothesis）。
该假说认为系统中所有物种对系统功能维持都具有虽然小但重要的作用，就像一架飞机上的一个个铆钉。
物种灭绝可以比拟为飞机失去铆钉，随着物种灭绝数量的增加，生态系统受损程度将逐渐加速地上升。
单调/驼峰模型假说（monotonic/unimodal hypothesis）是两种假说的复合体。
一种假说认为，多样性与生产力之间的关系是一种单调变化关系，即在生产力增加时，多样性也增加（但可能会达到一个极值）；另一种假说认为，多样性和生产力呈一种单峰曲线形式（状似驼峰,故也称为驼峰模型），即多样性在低水平时随生产力的增加而增加，但最终在达到足够高的生产力时反而降低。
Huston & DeAngelis （1994Huston M A, DeAngelis D L. Competition and coexistence:the effects of resource transport andsupply rates. American Naturalist, 1994, 144:954-957.）也认为多样性这种沿着生产力梯度格局的单峰曲线响应现象是普遍存在的。
Unit 1Where there is a will, there is a way. This proverb means that if you are really determined to do something, however difficult it might be, you will 1)eventually find a way to do it well. The 2) premier point is that you must have the will to achieve success. Ninety percent of the failures that occur are due to the fact that there is no strong will involved. Many people simply say that they want something, but they do not make any 3) endeavor to achieve it. So, instead of getting it, they use the poorest excuse to explain the situation away. On many occasions, people tend to 4) bypass every minute obstacle, making the objective impossible to attain. In reality, if they have the will to succeed, they can get rid of the 5) handicaps and achieve their goals. Only those with a(n) 6) committed and focused will and spirit can fight their way to final victory. Many a famous man has the same experience. They have 7) attained their prestige because they have had the will to 8) transcend apparently insuperable (无法克服的) obstacles. Many artists, statesmen, writers and inventors have manged to succeed because they possess a fierce will, which has helped them to accomplish major 9) feats. Therefore, we can see that the main thing which one needs is a strong will. Weak-willed people never climb to the top. They collapse at the 10) slightest use of force against them. Strong-willed people, on the other hand will stand up against all odds and will make it a point to succeed.Unit 2Social anxiety symptoms often begin during adolescence. It's a developmental process that is 1) characterized by profound psychological changes, especially in terms of how we relate to others. One of the most frustrating 2) aspects of the adolescent years is the tendency for self-focus and a decrease in the 3) amount of focus we have for the feelings and needs of others. While these changes are fairly universal, those of us who were born with a shy temperament (性格) can carry the adolescent fears, which may never 4) recede, into adulthood. An anxious temperament causes our brains to react forcibly when 5) exposed to the stress of sudden awareness of our peers and gradually we become more and more vulnerable. Our brains label the fear of exposure or embarrassment as highly dangerous. This may result in a(n) 6) vicious circle for many years: excessive self-consciousness and inhibition when you feel you are being observed. To cope with the problem, I would like to 7) challenge you to strive for increased focus on other people, in place of your 8) excessive focus on yourself. Yes, I know, this is easier said than done. The fear may cause you to feel that you will lose control or make a fool of yourself when you are in the spotlight (聚光灯). But if you begin to build a new response, in 9) reaction to your fears, you will gradually build up a stronger and more positive response. Remember, don't let self-consciousness 10) paralyze you! Be courageous!Unit 3Audrey Hepburn was a beautiful actress and model, who became one of the most successful and well-known actresses in the film 1) domain. She was a fashion icon and role model for women all over the world, helping to 2) define a particular type of fresh, vulnerable, elfin (小精灵似的) beauty. Today's 3) popularity of the slim fashion model is due to Audrey Hepburn's influence.Although she appeared frail (脆弱的), she was 4) mentally strong. At the end of her acting career when she entered a(n)5) diplomatic career as a Goodwill Ambassador for UNICEF, she was so solidly 6) committed to her cause that she was held in highest esteem (尊重) by even the most hardened politicians. Audrey originally started working for UNICEF in 1954, doing radio presentations. She always said it was happy to 7) devote her life to helping impoverished (穷困的) children after her own good fortune in 8) surviving the hardship of the Nazi occupation of Holland. She began her permanent ambassadorship in 1988 and 9) embarked on trips to many countries. She was always positive: "People in these places don't know Audrey Hepburn, but they recognize the name UNICEF. When they see UNICEF their faces light up, because they know that something is happening." In 1992, her 10) humanitarian work with those in need was recognized when she was awarded the Jean Hersholt Humanitarian Award by the Academy of Motion Picture Arts and Sciences for her contribution to humanity.Unit 4"Why would you leave behind your comfortable bed, your home, your family and friends? Why do you want to go alone?" When you are living out of a backpack for a period of time, you may often get questioned why. I seldom get time to sit down and 1) contemplate the reason I travel, but I believe that you only begin to truly live once you step outside of your 2) comfort zone. My first overseas trip was at 14 years old, which 3) sparked my curiosity for the world. Since finishing high school I have 4) ventured through various countries and been amazed by all the 5) diverse cultures scattered around the globe. My eyes are my greatest asset as they have 6) witnessed the most beautiful scenes that replay in my mind every day: 7) stunning landscapes, friendly locals, breath-taking architecture, and food that makes your mouth water once your eyes catch a(n) 8) glimpse. Traveling teaches you to be independent in the most 9) positive way. I know how to depend on myself, go out and meet people, and not let anyone else's expectations 10) dictate my life. Every day I see my dream and every day it's in a new place. I am 22 years young. I quite agree with Anthony Bourdain, "If you're 22, physically fit, hungry to learn and be better, I urge you to travel. Find out how other people live and eat and cook. Learn from them, wherever you go."Unit 5In a study conducted in the UK, it was found that only four out of every five 1) employees were happy at work. Surprisingly, contrary to popular 2) notion, friendly, supportive colleagues and a good manager, instead of the salary or the love for the work, have been found to be the 3) primary causes of happiness at work. So, how do you keep your spirits up and, at the same time, 4) foster a sense of joy on the job? Here is one of the tips to help you on your way to finding happiness and complete job satisfaction in the workplace. Start with a positive outlook. Happiness is a state of mind; it 5) reflects an attitude, though not many people realize it. Staying happy at work is totally based on your 6) motivations and on a positive outlook toward your job, not on 7) monetary rewards or material gain. Dwelling on (老是想着) the good 8) aspects of the work rather than A witnessed F diverse K sparked B dense G stubborn L positive C stunning H glimpse M comfort D cluster I ventured N dictate E contemplate J extended O tropical rattlingon and on (对...喋喋不休) about what makes you unhappy is the basic key to happiness. Negativity and 9) gossiping about bad things may be easy, but it is looking at the bright side that makes for the challenging part of a job. As Francesca Reigler puts it, "Happiness is an attitude. We either make ourselves 10) miserable, or happy and strong. The amount of work is the same."Unit 6World War II was a global war that was under way by 1939, and ended in 1945. It involved a vast majority of the world's nations, including all of the great powers, eventually forming two opposing military alliances: the Allies and the Axis(轴心国). It was the most 1) appalling and widespread war in human history, with 2) innumerable people serving in military units. In a state of"total war", the major participants placed their entire economic, industrial, and scientific capabilities at the service of the war effort, erasing(消除) the 3) distinction between civilian and military resources. Estimates for the total number of 4) casualties of the war vary, because many deaths went 5) unrecorded . Most suggest that some 60 million people died in the war. Many civilians died because of disease, starvation, and 6) massacres . The war ended with the total victory of the Allies over the Axis in 1945. World War II altered the political alignment(结盟) and social structure of the world. The United Nations was established to 7) foster international cooperation and prevent future conflicts. The Soviet Union and the United States emerged as rival superpowers, setting the 8) stage for the so-called Cold War, which lasted for the next 46 years. Meanwhile, the influence of the European great powers started to 9) decline , while the decolonization(非殖民地化) of Asia and Africa began. Most countries whose industries had been damaged moved toward economic recovery. Political integration, especially in Europe, emerged as an effort to 10) stabilize post-war relations.。
