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   62262篇
  免费   263篇
  国内免费   552篇
系统科学   1243篇
丛书文集   684篇
教育与普及   296篇
理论与方法论   522篇
现状及发展   34685篇
研究方法   1015篇
综合类   22441篇
自然研究   2191篇
  2013年   800篇
  2012年   662篇
  2011年   2745篇
  2009年   620篇
  2008年   894篇
  2007年   1019篇
  2006年   1055篇
  2005年   1243篇
  2004年   2304篇
  2003年   1971篇
  2002年   1652篇
  2001年   1607篇
  2000年   1171篇
  1999年   1037篇
  1998年   651篇
  1997年   774篇
  1996年   533篇
  1994年   685篇
  1993年   687篇
  1992年   999篇
  1991年   859篇
  1990年   954篇
  1989年   771篇
  1988年   735篇
  1987年   749篇
  1986年   801篇
  1985年   996篇
  1984年   844篇
  1983年   731篇
  1982年   827篇
  1981年   882篇
  1980年   983篇
  1979年   1651篇
  1978年   1440篇
  1977年   1477篇
  1976年   1225篇
  1975年   1207篇
  1974年   1286篇
  1973年   1406篇
  1972年   1474篇
  1971年   1548篇
  1970年   1733篇
  1969年   1549篇
  1968年   1462篇
  1967年   1369篇
  1966年   1141篇
  1965年   900篇
  1959年   546篇
  1958年   740篇
  1957年   598篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
71.
Summary The tissue specificity of lactate dehydrogenase (EC 1.1.1.27) in brain and eye of the teleostPhallichthys amates was examined by acrylamide gel electrophoresis. It is suggested that subunit association is a function of gene product accessibility superimposed upon genetic restriction of assembly.  相似文献   
72.
Summary The administration of 15(R)-15-methyl prostaglandin E2 (15(R)-15-M-PGE2) in vivo significantly diminished the uptake of59Fe into blood, spleen, liver, femur and dried intestine of rats, whereas acetylsalicylic acid (ASA) increased the counts significantly. This effect of ASA was counteracted by 15(R)-15-M-PGE2. It is suggested that prostaglandins (PGs) might play an important role in inhibiting iron absorption at the intestinal level.This work was supported by grant No.6638 from CONICET (Argentina). The technical assistance of Mrs María E. Castro and Norma Rizzo is gratefully acknowleged.  相似文献   
73.
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   
74.
Summary The nature of antigen Dd, an antigen present in the extracts of human dandruff which precipitates human sera selectively, and antibodies reacting with it are reported.This work was supported through U.G.C. Grant No. F.23-230/75/SR II. H.K. participated in this study, first as J.R.F. of U.G.C. and then as S.R.F. of I.C.M.R. We are grateful to Dr Baruch S. Blumberg for his invaluable suggestions and to Professor H. Walter for the gift of IgG, IgM and IgA immune sera.  相似文献   
75.
We discuss a generalization of the standard notion of probability space and show that the emerging framework, to be called operational probability theory, can be considered as underlying quantal theories. The proposed framework makes special reference to the convex structure of states and to a family of observables which is wider than the familiar set of random variables: it appears as an alternative to the known algebraic approach to quantum probability.  相似文献   
76.
金属基复合材料界面层阻尼功能研究   总被引:4,自引:0,他引:4  
采用CVD技术制备了具有不同界面层的C-f/Al金属基复合材料,获得了一种界面层阻尼功能设计的新方法. 研究发现具有特殊界面层的C-f/Al复合材料的抗拉强度、弹性模量和阻尼性能比无界面层时都有明显增加,并且不同界面层的效果不同. 碳层对复合材料阻尼性能的提高效果最大,硅层的提高效果不如碳层,碳硅混合层的效果居中. 涂层的厚度也影响了阻尼提高的效果,较厚的碳层效果更好,这是由于提高了复合材料的阻尼应变振幅效应而产生的. 研究认为发生在界面层的微滑移是其主要的阻尼机制.  相似文献   
77.
78.
A detailed karyotypic analysis of two Egyptian species of bats,Taphozous perforatus andTaphozous nudiventris, was made on the basis of conventional data and G-band patterns. No detectable karyotypic difference was found between the two species (2n=42, F.N. 64). By comparison of G-band patterns, some differences between the two species were seen in the corresponding autosomal pairs. These results are reported for the first time in Egypt.  相似文献   
79.
80.
Recent years have seen an increasing cross-fertilization between the fields of decision analysis and forecasting. Decision-analytic models often require forecasts as inputs, and aspects of the Bayesian decision-theoretic framework underlying decision analysis have proved useful to forecasting, particularly in contexts where subjective judgemental inputs are required. This paper describes the use of decision tree analysis for forecasting and illustrates its use for corporate divisional forecasting and planning. A specialized decision-analytic technique, acts as events, is also described and illustrated to forecast a new product's earnings. Conclusions are drawn about the applicability of decision analysis for forecasting.  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