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   34481篇
  免费   84篇
  国内免费   123篇
系统科学   179篇
丛书文集   788篇
教育与普及   126篇
理论与方法论   206篇
现状及发展   14990篇
研究方法   1418篇
综合类   16602篇
自然研究   379篇
  2013年   193篇
  2012年   462篇
  2011年   882篇
  2010年   209篇
  2009年   209篇
  2008年   609篇
  2007年   620篇
  2006年   644篇
  2005年   668篇
  2004年   613篇
  2003年   619篇
  2002年   623篇
  2001年   1091篇
  2000年   989篇
  1999年   691篇
  1992年   632篇
  1991年   528篇
  1990年   552篇
  1989年   539篇
  1988年   552篇
  1987年   556篇
  1986年   532篇
  1985年   665篇
  1984年   558篇
  1983年   450篇
  1982年   405篇
  1981年   390篇
  1980年   503篇
  1979年   1080篇
  1978年   958篇
  1977年   929篇
  1976年   641篇
  1975年   689篇
  1974年   1003篇
  1973年   863篇
  1972年   876篇
  1971年   1100篇
  1970年   1412篇
  1969年   1068篇
  1968年   1019篇
  1967年   1057篇
  1966年   895篇
  1965年   658篇
  1959年   382篇
  1958年   555篇
  1957年   472篇
  1956年   391篇
  1955年   328篇
  1954年   376篇
  1948年   201篇
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
31.
S J Weintraub  C A Prater  D C Dean 《Nature》1992,358(6383):259-261
  相似文献   
32.
33.
34.
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   
35.
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   
36.
This paper assesses the information content of two survey indicators for consumption developments in the near future for eight European countries in the period 1985–1998. Empirical work on this topic typically focuses on consumer confidence, the perceptions of buyers of consumption goods. This paper examines whether perceptions of sellers of consumption goods, measured by retail trade surveys, may also improve short‐term monitoring of consumption. We find that both consumer confidence and retailer confidence embody valuable information, when analysed in isolation. For France, Italy and Spain we conclude that adding retail confidence does not improve the indicator model once consumer confidence has been included. For the UK the reverse case is obtained. For the remaining four countries we show that combining consumer sentiment and retail trade confidence into a composite indicator leads to optimal results. Our results suggest that incorporating information from retail trade surveys may offer significant benefits for the analysis of short‐term prospects of consumption. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
37.
38.
Y Dan  M M Poo 《Nature》1992,359(6397):733-736
It is well known that transmitter secretion requires specialized secretory organelles, the synaptic vesicles, for the packaging, storage and exocytotic release of the transmitter. Here we report that when acetylcholine (ACh) is loaded into an isolated Xenopus myocyte, there is spontaneous quantal release of ACh from the myocyte which results in activation of its own surface ACh channels and the appearance of membrane currents resembling miniature endplate currents. This myocyte secretion probably reflects Ca(2+)-regulated exocytosis of ACh-filled cytoplasmic compartments. Furthermore, step depolarization of the myocyte membrane triggers evoked ACh release from the myocyte with a weak excitation-secretion coupling. These findings suggest that quantal transmitter secretion does not require secretory pathways unique to neurons and that the essence of presynaptic differentiation may reside in the provision of transmitter supply and modification of the preexisting secretion pathway.  相似文献   
39.
The characteristic circular dichroism of bilirubin bound to human serum albumin undergoes a remarkable sign inversion on addition of halothane, chloroform and other volatile anesthetics. This sign inversion, which is completely reversed by removal of the anesthetic, reflects a pronounced conformational change of the bound ligand; probably a complete inversion of chirality. The observation suggests that association of volatile anesthetics with proteins can markedly alter the internal topography of receptor sites and potentially influence the stereoselectivity of ligand binding.  相似文献   
40.
A linear regression model with random walk coefficients is extended to allow for linear restrictions between the coefficients to be satisfied at each point in time. Estimation in this model is shown to be no more involved than estimation in the standard model. It is also demonstrated how, after a slight modification to the testing problem, classical test procedures may be applied to the problem of testing for such restrictions. The performance of the Lagrange Multiplier test for a variety of different restrictions is then investigated via simulation. An empirical application involving testing for homogeneity in a random walk coefficient version of the AIDS model is given.  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