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   63234篇
  免费   310篇
  国内免费   583篇
系统科学   1292篇
丛书文集   403篇
教育与普及   314篇
理论与方法论   530篇
现状及发展   34143篇
研究方法   1237篇
综合类   24015篇
自然研究   2193篇
  2013年   834篇
  2012年   775篇
  2011年   2821篇
  2009年   665篇
  2008年   959篇
  2007年   1075篇
  2006年   1164篇
  2005年   1348篇
  2004年   2419篇
  2003年   2019篇
  2002年   1782篇
  2001年   1730篇
  2000年   1248篇
  1999年   1191篇
  1998年   687篇
  1997年   819篇
  1996年   579篇
  1994年   724篇
  1993年   725篇
  1992年   1087篇
  1991年   915篇
  1990年   991篇
  1989年   806篇
  1988年   765篇
  1987年   790篇
  1986年   851篇
  1985年   993篇
  1984年   877篇
  1983年   740篇
  1982年   844篇
  1981年   881篇
  1980年   984篇
  1979年   1638篇
  1978年   1430篇
  1977年   1400篇
  1976年   1223篇
  1975年   1219篇
  1974年   1187篇
  1973年   1354篇
  1972年   1433篇
  1971年   1533篇
  1970年   1708篇
  1969年   1404篇
  1968年   1359篇
  1967年   1298篇
  1966年   1088篇
  1965年   840篇
  1964年   517篇
  1958年   680篇
  1957年   558篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
This paper discusses the use of the Kast and Rosenzweig systems and contingency model for teaching and practice in organizational analysis and planning, management policy, and organizational development. The need for an organizational model for graduate students and field-based managers and executives is identified. The model is presented beginning with the Kast and Rosenzweig work, with integration of research by Daft, Schein, Trist, Deal and Kennedy, Ackoff, Delbecq, and Mintzberg. The teaching and practice uses of the model in four areas are presented, including organizational analysis and planning, management policy making, and organizational development. In each area case examples of the use of the model in teaching and practice are presented. Needs for future research and implications for the use of the model are discussed.  相似文献   
72.
S J Salser  C Kenyon 《Nature》1992,355(6357):255-258
Anterior-posterior patterning in insects, vertebrates and nematodes involves members of conserved Antennapedia-class homeobox gene clusters (HOM-C) that are thought to give specific body regions their identities. The effects of these genes on region-specific body structures have been described extensively, particularly in Drosophila, but little is known about how HOM-C genes affect the behaviours of cells that migrate into their domains of function. In Caenorhabditis elegans, the Antennapedia-like HOM-C gene mab-5 not only specifies postembryonic fates of cells in a posterior body region, but also influences the migration of mesodermal and neural cells that move through this region. Here we show that as one neuroblast migrates into this posterior region, it switches on mab-5 gene expression; mab-5 then acts as a developmental switch to control the migratory behaviour of the neuroblast descendants. HOM-C genes can therefore not only direct region-specific patterns of cell division and differentiation, but can also act within migrating cells to programme region-specific migratory behaviour.  相似文献   
73.
74.
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   
75.
本文合成了表面修饰的Mn2O3,超微粒子(UFP)有机溶胶,并采用简并的四波混频(DFWM)手段研究了Mn2O3有机溶胶的三阶非线性光学特性,得到了较大的非线性极化率X3.最后讨论了Mn2O3超微粒子的三阶非线性产生机制.  相似文献   
76.
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   
77.
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   
78.
S J Weintraub  C A Prater  D C Dean 《Nature》1992,358(6383):259-261
  相似文献   
79.
80.
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   
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