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   61069篇
  免费   153篇
  国内免费   240篇
系统科学   470篇
丛书文集   1393篇
教育与普及   144篇
理论与方法论   331篇
现状及发展   28077篇
研究方法   2127篇
综合类   27950篇
自然研究   970篇
  2013年   462篇
  2012年   767篇
  2011年   1770篇
  2010年   350篇
  2008年   1010篇
  2007年   1096篇
  2006年   1136篇
  2005年   1119篇
  2004年   1077篇
  2003年   1072篇
  2002年   1037篇
  2001年   1706篇
  2000年   1579篇
  1999年   1053篇
  1992年   1019篇
  1991年   844篇
  1990年   919篇
  1989年   895篇
  1988年   874篇
  1987年   937篇
  1986年   916篇
  1985年   1128篇
  1984年   892篇
  1983年   774篇
  1982年   672篇
  1981年   699篇
  1980年   900篇
  1979年   1883篇
  1978年   1611篇
  1977年   1593篇
  1976年   1219篇
  1975年   1321篇
  1974年   1801篇
  1973年   1593篇
  1972年   1677篇
  1971年   1958篇
  1970年   2533篇
  1969年   1927篇
  1968年   1861篇
  1967年   1908篇
  1966年   1629篇
  1965年   1182篇
  1964年   353篇
  1959年   713篇
  1958年   1145篇
  1957年   846篇
  1956年   736篇
  1955年   640篇
  1954年   720篇
  1948年   450篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Summary Hydrochlorothiazide, acutely injected in rats, has a weak analgesic activity per se and potentiates and prolongs the antinociceptive effect of morphine.This work was supported in part by grants from Consiglio Nazionale delle Ricerche, and by Ministero della Pubblica Istruzione, Roma.  相似文献   
72.
73.
74.
Summary Traces of nor-adrenaline restore the vascular action of adrenaline altered in epinephrectomized dogs to the reaction of the normal animal. Therefore it is claimed that the adrenals discharge one or several substances into the blood stream, which are necessary for the usual peripheral vascular action of adrenaline. Further investigations are in progress.  相似文献   
75.
76.
1 Results The development of the concepts of nanotechnology has given an important impact on the design of new polymer based materials which are in most cases characterized by a multiphase morphology. When at least one phase has nanometric dimension(s) the system can be considered as a nanocomposite where the interface is not only determining for the adhesion but also may play a role in some bulk properties. Indeed in nanostructured multiphase solids the interface is significant as a bulk component. The...  相似文献   
77.
1 Results Pyrimidinones (PMs) are a class of important heterocycles which have been well documented throughout the literature due to their biological importance. They exhibit a wide range of pharmaceutical and therapeutic properties[1].A rapid and efficient one-pot method for the synthesis of 4,6-diarylpyrimidin-2(1H)- ones and related heterocycles is described.The condensation of acetophenone derivatives,aldehydes and urea in the presence of sulfamic acid was employed to synthesize a variety of pyrimid...  相似文献   
78.
Summary The application of diphenylilacetic acid has shown favorable effects on the growth of tobacco plants. Solutions containing 10 p.p.m. and 20 p.p.m. produce, in weight, an almost double growth in treated plants in comparison with the controls. Diphenylilacetic acid can be considered as a growth-promoting substance. The data are not definite.  相似文献   
79.
1 Introduction   Nanoscale science and technology have attracted greatattention since the novel properties become dominant for well-known materials as their sizes reduced to some critical dimension. These properties frequently originate in lattice distortions, structure transformations etc[1]. The variations of size and structure ofnanocrystals lead to the change of their electronic structures. Very recently, nanoscale materials with different electronic structures were utilized in fabricating nanoscale junctions[2].   ……  相似文献   
80.
In econometrics, as a rule, the same data set is used to select the model and, conditional on the selected model, to forecast. However, one typically reports the properties of the (conditional) forecast, ignoring the fact that its properties are affected by the model selection (pretesting). This is wrong, and in this paper we show that the error can be substantial. We obtain explicit expressions for this error. To illustrate the theory we consider a regression approach to stock market forecasting, and show that the standard predictions ignoring pretesting are much less robust than naive econometrics might suggest. We also propose a forecast procedure based on the ‘neutral Laplace estimator’, which leads to an improvement over standard model selection procedures. Copyright © 2004 John Wiley & Sons, Ltd.  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