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   63273篇
  免费   285篇
  国内免费   554篇
系统科学   1245篇
丛书文集   522篇
教育与普及   280篇
理论与方法论   524篇
现状及发展   34052篇
研究方法   1260篇
综合类   24042篇
自然研究   2187篇
  2013年   818篇
  2012年   735篇
  2011年   2854篇
  2009年   618篇
  2008年   940篇
  2007年   1124篇
  2006年   1138篇
  2005年   1357篇
  2004年   2527篇
  2003年   2054篇
  2002年   1764篇
  2001年   1692篇
  2000年   1197篇
  1999年   1162篇
  1998年   644篇
  1997年   778篇
  1996年   537篇
  1994年   697篇
  1993年   694篇
  1992年   1090篇
  1991年   879篇
  1990年   967篇
  1989年   788篇
  1988年   758篇
  1987年   776篇
  1986年   847篇
  1985年   1030篇
  1984年   856篇
  1983年   746篇
  1982年   824篇
  1981年   871篇
  1980年   968篇
  1979年   1609篇
  1978年   1414篇
  1977年   1398篇
  1976年   1199篇
  1975年   1186篇
  1974年   1206篇
  1973年   1356篇
  1972年   1460篇
  1971年   1496篇
  1970年   1753篇
  1969年   1513篇
  1968年   1414篇
  1967年   1319篇
  1966年   1150篇
  1965年   912篇
  1964年   527篇
  1958年   775篇
  1957年   608篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
231.
232.
Q Gu  F A Popp 《Experientia》1992,48(11-12):1069-1082
By considering an exciplex system consisting of collective molecules in interaction with both the 'pumping' fields and the biophoton fields, the two-level exciplex model and the three-level exciplex model are presented. They are useful for the investigation of the quasi-stationary behaviour of biophoton emission, and biophoton emission as a dynamic process in the presence of external perturbations. Our theoretical results predict a series of nonlinear effects, such as chaos, fractal behaviour, and non-equilibrium phase transition. These effects characterize the coherence nature of living systems. In our approaches, there are two important quantities f and x, which can be used to mark the working points of the two-level and three-level exciplex systems. All the influences of external perturbations on the exciplex systems, e.g. change of temperature, the addition of agents, exposure to light, etc., can be interpreted as shifts of the working points of the systems, leading to a diversity of nonlinear response of biophoton emission. In addition, the agreements of the theoretical results and the corresponding experimental observations on biophoton emission from biological systems in the presence of external perturbations are demonstrated.  相似文献   
233.
This paper is concerned with the development of ‘Systems Thinking.’ In particular, it considers and critiques ‘traditional Systems Thinking’ within the framework of Critical Systems Thinking. Before embarking on such a venture it is necessary to derive a base context from which to develop argument and analysis. Therefore the outline of this paper is as follows: A contextual setting for Systems Thinking, Theoretical Considerations, Soft Systems Thinking and Methodology, and A Critical Systems Thinking (CST) approach. The account of the development of Systems Thinking is followed by a synopsis of a theoretical framework for Systems Thinking which will allow us to gain an understanding of contemporary views. The paper then goes on to review SSM and CST in order to provide a platform for a critique of traditional forms of Systems Thinking. The final section briefly discusses the applicability of CST to the ‘real-world’ context by outlining some current studies being undertaken by the author.  相似文献   
234.
This paper introduces the conception of open complex giant system and the methodology for dealing with the system, with stress on its profound significance in development of science and technology. The authors conclude that the reductionism underlying the exact science is not suitable to open complex giant system, and the only feasible alternative is the meta-synthetic engineering from the qualitative to the quantitative.  相似文献   
235.
After a brief recall of the Sidelobe Canceler (SLC) working principle, including the derivation of a general formula for the Cancellation Ratio (CR), the effects of channel mismatching are investigated. In particular, curves providing CRvalues as a function of amplitude and phase channel mismatching, radar bandwidth, and jammer direction of arrival (JDOA) are provided for the cases of one and two auxiliary antennas. Subsequently, a time -space processor for performance restoration is analyzed in detail. In addition to the above mentioned quantities, the attainable CR value is expressed as a function of the space-time processor parameters. The contribution of the paper is related to the derivation of a number of mathematical equations of CR for several cases of practical interest for the radar engineer. In addition, several curves are presented to assist design of SLC systems.  相似文献   
236.
237.
等截面垂直管道中气粉流的临界管长   总被引:2,自引:1,他引:1  
考虑重力和摩擦力的影响,建立了描述垂直管道中气粉流行为的数学模型,采用马赫数检验法,确定了产生摩擦壅塞时的临界管长,计算表明,当气粉流以不同方式喷入相同的冶金熔池时,底吹垂直管道比顶吹垂直管道需要更大的临界管长。  相似文献   
238.
239.
240.
M McClelland  A S Bhagwat 《Nature》1992,355(6361):595-596
  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