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   55970篇
  免费   272篇
  国内免费   549篇
系统科学   1253篇
丛书文集   190篇
教育与普及   288篇
理论与方法论   524篇
现状及发展   31740篇
研究方法   871篇
综合类   19717篇
自然研究   2208篇
  2013年   810篇
  2012年   620篇
  2011年   2669篇
  2009年   617篇
  2008年   827篇
  2007年   942篇
  2006年   1032篇
  2005年   1183篇
  2004年   2278篇
  2003年   1937篇
  2002年   1584篇
  2001年   1511篇
  2000年   1053篇
  1999年   1020篇
  1998年   647篇
  1997年   777篇
  1996年   534篇
  1994年   687篇
  1993年   694篇
  1992年   946篇
  1991年   807篇
  1990年   900篇
  1989年   691篇
  1988年   650篇
  1987年   684篇
  1986年   758篇
  1985年   896篇
  1984年   784篇
  1983年   686篇
  1982年   808篇
  1981年   829篇
  1980年   904篇
  1979年   1417篇
  1978年   1252篇
  1977年   1231篇
  1976年   1117篇
  1975年   1079篇
  1974年   932篇
  1973年   1178篇
  1972年   1207篇
  1971年   1242篇
  1970年   1323篇
  1969年   1203篇
  1968年   1186篇
  1967年   1050篇
  1966年   889篇
  1965年   737篇
  1964年   485篇
  1958年   526篇
  1957年   435篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
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   
72.
73.
S P Wolff 《Nature》1992,356(6368):375-376
The introduction of human insulin to treat diabetics seemed straightforward. What can account for the problems that have followed?  相似文献   
74.
75.
Vitamin B12 (methylcobalamin) was administered orally (3 mg/day) to 9 healthy subjects for 4 weeks. Nocturnal melatonin levels after exposure to bright light (ca. 2500 lx) were determined, as well as the levels of plasma melatonin over 24 h. The timing of sleep was also recorded. Vitamin B12 was given blind to the subjects and crossed over with placebo. We found that the 24-h melatonin rhythm was significantly phase-advanced (1.1 h) in the vitamin B12 trial as compared with that in the placebo trial. In addition, the 24-h mean of plasma melatonin level was much lower in the vitamin B12 trial than with the placebo. Furthermore, the nocturnal melatonin levels during bright light exposure were significantly lower in the vitamin B12 trial than with the placebo. On the other hand, vitamin B12 did not affect the timing of sleep. These findings raise the possibility that vitamin B12 phase-advances the human circadian rhythm by increasing the light sensitivity of the circadian clock.  相似文献   
76.
Retrograde transport of endocytosed Shiga toxin to the endoplasmic reticulum.   总被引:39,自引:0,他引:39  
K Sandvig  O Garred  K Prydz  J V Kozlov  S H Hansen  B van Deurs 《Nature》1992,358(6386):510-512
Shiga toxin and some other protein toxins that act on targets in the cytosol have previously been shown to enter the trans-Golgi network. Transport by this route may be necessary for translocation of the toxin to the cytosol and for intoxication, but it is not known whether the enzymatically active part of the toxins actually enters the cytosol from the trans-Golgi network. It has been suggested that such toxins are transported in a retrograde manner to the endoplasmic reticulum and that translocation occurs in this organelle, but retrograde transport of endocytosed material beyond the trans-Golgi network has never been demonstrated. Here we show that in butyric acid-treated A431 cells endocytosed Shiga toxin is not only transported to the trans-Golgi network, but also to all Golgi stacks, to the endoplasmic reticulum and to the nuclear envelope. Furthermore, butyric acid sensitizes the cells to Shiga toxin, which is consistent with the possibility that retrograde transport is required for translocation of the toxin to the cytosol.  相似文献   
77.
S J Weintraub  C A Prater  D C Dean 《Nature》1992,358(6383):259-261
  相似文献   
78.
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   
79.
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   
80.
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