首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5405篇
  免费   27篇
  国内免费   29篇
系统科学   89篇
丛书文集   2篇
教育与普及   11篇
理论与方法论   94篇
现状及发展   2011篇
研究方法   548篇
综合类   2548篇
自然研究   158篇
  2020年   18篇
  2019年   17篇
  2018年   73篇
  2017年   51篇
  2016年   73篇
  2015年   52篇
  2014年   59篇
  2013年   80篇
  2012年   296篇
  2011年   495篇
  2010年   133篇
  2009年   33篇
  2008年   311篇
  2007年   306篇
  2006年   314篇
  2005年   339篇
  2004年   339篇
  2003年   261篇
  2002年   310篇
  2001年   54篇
  2000年   51篇
  1999年   44篇
  1996年   19篇
  1992年   29篇
  1991年   24篇
  1990年   32篇
  1989年   22篇
  1987年   31篇
  1986年   21篇
  1985年   29篇
  1984年   31篇
  1983年   20篇
  1982年   39篇
  1981年   28篇
  1980年   35篇
  1979年   97篇
  1978年   79篇
  1977年   99篇
  1976年   23篇
  1975年   39篇
  1974年   100篇
  1973年   88篇
  1972年   105篇
  1971年   77篇
  1970年   94篇
  1969年   96篇
  1968年   70篇
  1967年   95篇
  1966年   77篇
  1965年   59篇
排序方式: 共有5461条查询结果,搜索用时 406 毫秒
1.
国际空间站和欧洲核子中心的大型强子对撞机 (LHC)计划目前都面临资金上的窘境 ;……除了对项目 (尤其是耗资巨大的项目 )预算的科学化 ,管理的科学化在其中尤显重要  相似文献   
2.
I offer a reply to criticisms of the Strong Programme presented by Stephen Kemp who develops some new lines of argument that focus on the ‘monism’ of the programme. He says the programme should be rejected for three reasons. First, because it embodies ‘weak idealism’, that is, its supporters effectively sever the link between language and the world. Second, it challenges the reasons that scientists offer in explanation of their own beliefs. Third, it destroys the distinction between successful and unsuccessful instrumental action. Kemp is careful to produce quotations from the supporters of the programme as evidence to support his case. All three points deserve and are given a detailed response and the interpretation of the quoted material plays a significant role in the discussion. My hope is that careful exegesis will offset the numerous misinterpretations that are current in the philosophical literature. Particular attention is paid to what is said about the normative standards involved in the application of empirical concepts. The operation of these standards in the face of the negotiability of all concepts is explored and misapprehensions on the topic are corrected. The work of Wittgenstein, Popper, Kuhn and Hesse is used to illustrate these themes.  相似文献   
3.
Duhem’s concept of ‘good sense’ is central to his philosophy of science, given that it is what allows scientists to decide between competing theories. Scientists must use good sense and have intellectual and moral virtues in order to be neutral arbiters of scientific theories, especially when choosing between empirically adequate theories. I discuss the parallels in Duhem’s views to those of virtue epistemologists, who understand justified belief as that arrived at by a cognitive agent with intellectual and moral virtues, showing how consideration of Duhem as a virtue epistemologist offers insights into his views, as well as providing possible answers to some puzzles about virtue epistemology. The extent to which Duhem holds that the intellectual and moral virtues of the scientist determine scientific knowledge has not been generally noticed.  相似文献   
4.
Linear models are invariant under non-singular, scale-preserving linear transformations, whereas mean square forecast errors (MSFEs) are not. Different rankings may result across models or methods from choosing alternative yet isomorphic representations of a process. One approach can dominate others for comparisons in levels, yet lose to another for differences, to a second for cointegrating vectors and to a third for combinations of variables. The potential for switches in ranking is related to criticisms of the inadequacy of MSFE against encompassing criteria, which are invariant under linear transforms and entail MSFE dominance. An invariant evaluation criterion which avoids misleading outcomes is examined in a Monte Carlo study of forecasting methods.  相似文献   
5.
The visual field is topographically mapped onto the primary visual cortex (V1), forming a retinotopic map which is far more detailed for the foveal regions than for the periphery. We found that receptive field (RF) size in monkey V1 increases with eccentricity, and that a 1—2 m2 V1 region contains roughly a complete set of machinery necessary to analyze a visual-field area whose size is about that of the RFs of the cells. This allows V1 to be anatomically uniform, and is in sharp contrast with the retina.  相似文献   
6.
A physically based model for ground‐level ozone forecasting is evaluated for Santiago, Chile. The model predicts the daily peak ozone concentration, with the daily rise of air temperature as input variable; weekends and rainy days appear as interventions. This model was used to analyse historical data, using the Linear Transfer Function/Finite Impulse Response (LTF/FIR) formalism; the Simultaneous Transfer Function (STF) method was used to analyse several monitoring stations together. Model evaluation showed a good forecasting performance across stations—for low and high ozone impacts—with power of detection (POD) values between 70 and 100%, Heidke's Skill Scores between 40% and 70% and low false alarm rates (FAR). The model consistently outperforms a pure persistence forecast. Model performance was not sensitive to different implementation options. The model performance degrades for two‐ and three‐days ahead forecast, but is still acceptable for the purpose of developing an environmental warning system at Santiago. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
7.
A Vincent  D Heitz  C Petit  C Kretz  I Oberlé  J L Mandel 《Nature》1991,349(6310):624-626
The fragile-X syndrome is the most frequent inherited form of mental retardation, with an incidence of 1 in 1,500 males. It is characterized by the presence of a fragile site at Xq27.3 induced in vitro by folate deprivation or by inhibitors of deoxynucleotide synthesis. Its mode of inheritance is unusual for an X-linked trait, with incomplete penetrance in both males and females. Some phenotypically normal males transmit the mutation to all their daughters who rarely express any symptoms, but penetrance is high in sons and daughters of these carrier women. Genetic and physical mapping of the Xq27-q28 region has confirmed that the disease locus is located at or very near the fragile site. Hypotheses proposed to account for the abnormalities in the inheritance of the disease include sequence rearrangements by meiotic recombination or a mutation that affects reactivation of an inactive X chromosome during differentiation of female germ cells. To detect such rearrangements, or methylation changes that may reflect a locally inactive X chromosome, we used pulsed-field gel analysis of DNA from fragile-X patients with probes close to the fragile-X locus. The probe Do33 (DXS465) detected abnormal patterns in fragile-X patients, but not in normal controls or in non-expressing male transmitters.  相似文献   
8.
Summary Host cell reactivation capacity for ozone T3 phage was determined for differentE. coli strains deficient in one or more of the DNA repair mechanisms. The results indicate that DNA polymerase I appears to play a key role in the repair of damage produced on the DNA by ozone while thelexA gene product seems to play a minor one.This research was sponsored by the National Sciences and Engineering Council of Canada. One of us (PLH) acknowledges a scholarship from the NSERCof Canada.  相似文献   
9.
Chromosomal effects of adeno-associated virus vector integration.   总被引:17,自引:0,他引:17  
Adeno-associated virus (AAV) vectors are currently being used in several clinical gene-therapy trials (see the NIH OBA Human Gene Transfer Clinical Trials Database); however, little is known about the chromosomal effects of vector integration. Here we report that integrated vector proviruses are associated with chromosomal deletions and other rearrangements and are frequently located on chromosome 19 (although not at the wildtype AAV integration site).  相似文献   
10.
During the evolution of proteins the pressure to optimize biological activity is moderated by a need for efficient folding. For most proteins, this is accomplished through spontaneous folding to a thermodynamically stable and active native state. However, in the extracellular bacterial alpha-lytic protease (alphaLP) these two processes have become decoupled. The native state of alphaLP is thermodynamically unstable, and when denatured, requires millennia (t1/2 approximately 1,800 years) to refold. Folding is made possible by an attached folding catalyst, the pro-region, which is degraded on completion of folding, leaving alphaLP trapped in its native state by a large kinetic unfolding barrier (t1/2 approximately 1.2 years). alphaLP faces two very different folding landscapes: one in the presence of the pro-region controlling folding, and one in its absence restricting unfolding. Here we demonstrate that this separation of folding and unfolding pathways has removed constraints placed on the folding of thermodynamically stable proteins, and allowed the evolution of a native state having markedly reduced dynamic fluctuations. This, in turn, has led to a significant extension of the functional lifetime of alphaLP by the optimal suppression of proteolytic sensitivity.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号