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   72402篇
  免费   293篇
  国内免费   564篇
系统科学   1261篇
丛书文集   729篇
教育与普及   292篇
理论与方法论   536篇
现状及发展   36701篇
研究方法   1639篇
综合类   29903篇
自然研究   2198篇
  2013年   818篇
  2012年   848篇
  2011年   3008篇
  2009年   643篇
  2008年   1064篇
  2007年   1310篇
  2006年   1294篇
  2005年   1526篇
  2004年   2681篇
  2003年   2218篇
  2002年   1972篇
  2001年   2016篇
  2000年   1503篇
  1999年   1324篇
  1998年   654篇
  1997年   783篇
  1994年   702篇
  1993年   697篇
  1992年   1256篇
  1991年   1040篇
  1990年   1136篇
  1989年   940篇
  1988年   899篇
  1987年   921篇
  1986年   991篇
  1985年   1238篇
  1984年   989篇
  1983年   861篇
  1982年   892篇
  1981年   926篇
  1980年   1064篇
  1979年   1933篇
  1978年   1640篇
  1977年   1615篇
  1976年   1370篇
  1975年   1372篇
  1974年   1499篇
  1973年   1561篇
  1972年   1693篇
  1971年   1791篇
  1970年   2178篇
  1969年   1793篇
  1968年   1727篇
  1967年   1618篇
  1966年   1443篇
  1965年   1117篇
  1959年   629篇
  1958年   1004篇
  1957年   762篇
  1956年   642篇
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
81.
In this paper two alternative loss criteria for the least squares Procrustes problem are studied. These alternative criteria are based on the Huber function and on the more radical biweight function, which are designed to be resistant to outliers. Using iterative majorization it is shown how a convergent reweighted least squares algorithm can be developed. In asimulation study it turns out that the proposed methods perform well over a specific range of contamination. When a uniform dilation factor is included, mixed results are obtained. The methods also yield a set of weights that can be used for diagnostic purposes.  相似文献   
82.
Studies of intracellular traffic in yeast and mammalian systems have implicated members of the Rab family of small GTP-binding proteins as regulators of membrane fusion. We have used the patch clamp technique to measure exocytotic fusion events directly and investigate the role of GTP-binding proteins in regulating exocytosis in mast cells. Intracellular perfusion of mast cells with GTP-gamma S is sufficient to trigger complete exocytotic degranulation in the absence of other intracellular messengers. Here we show that GTP is a potent inhibitor of GTP-gamma S-induced degranulation, indicating that sustained activation of a GTP-binding protein is sufficient for membrane fusion. We have found that synthetic oligopeptides, corresponding to part of the effector domain of Rab3a, stimulate complete exocytotic degranulation, similar to that induced by GTP-gamma S. The response is selective for Rab3a sequence and is strictly dependent on Mg2+ and ATP. This suggests that sustained activation of a Rab3 protein causes exocytotic fusion. The peptide response can be accelerated by GDP-beta S, suggesting that Rab3a peptides compete with endogenous Rab3 proteins for a binding site on a target effector protein, which causes fusion on activation.  相似文献   
83.
84.
S J Weintraub  C A Prater  D C Dean 《Nature》1992,358(6383):259-261
  相似文献   
85.
通过对“洋务运动博物馆”东立面形象的反思 ,讨论了建筑形态设计中隐喻手法的运用 ;就此引申 ,探讨传统问题 ,指出正确理解传统与创新的四个方面。  相似文献   
86.
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   
87.
88.
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   
89.
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   
90.
1 Results This paper presents the development of platinum nanocomposites structures based on organically modified c.a.2 nm core platinum nanoparticles.The chemical modification of the 4-mercaptoaniline functionalized particles by various in coming molecules is evidenced and precisely quantified.The particles can be dissolved like molecules in various solvents depending on the features of the new crown and X-rays shows that the interparticle distance is affected by the crown modification.These platinum n...  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