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   44897篇
  免费   143篇
  国内免费   155篇
系统科学   372篇
丛书文集   868篇
教育与普及   79篇
理论与方法论   192篇
现状及发展   20235篇
研究方法   1751篇
综合类   20897篇
自然研究   801篇
  2013年   328篇
  2012年   642篇
  2011年   1427篇
  2010年   281篇
  2008年   743篇
  2007年   870篇
  2006年   844篇
  2005年   874篇
  2004年   924篇
  2003年   816篇
  2002年   786篇
  2001年   1280篇
  2000年   1223篇
  1999年   809篇
  1992年   809篇
  1991年   638篇
  1990年   685篇
  1989年   618篇
  1988年   623篇
  1987年   658篇
  1986年   657篇
  1985年   891篇
  1984年   647篇
  1983年   536篇
  1982年   478篇
  1981年   506篇
  1980年   642篇
  1979年   1423篇
  1978年   1118篇
  1977年   1089篇
  1976年   868篇
  1975年   933篇
  1974年   1312篇
  1973年   1111篇
  1972年   1209篇
  1971年   1346篇
  1970年   1825篇
  1969年   1435篇
  1968年   1233篇
  1967年   1304篇
  1966年   1183篇
  1965年   896篇
  1964年   273篇
  1959年   493篇
  1958年   855篇
  1957年   611篇
  1956年   492篇
  1955年   452篇
  1954年   500篇
  1948年   337篇
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
141.
142.
143.
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   
144.
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   
145.
The effects of alcohol consumption, cigarette smoking and age on sister chromatid exchange (SCE) frequency in human lymphocytes were assessed by means of multiple linear regression. An increase in SCE rates was associated with alcohol consumption (p = 0.0001), smoking (p = 0.0231), and, to a small extent (p = 0.057), age. These three confounding factors explain 48% of the inter-personal variation in SCE rates among subjects studied.  相似文献   
146.
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   
147.
148.
以B2O3粉扩NH3为原料,用脉冲等离子体反应器合成BN,所得产品经X射线衍射,红外光谱分析表明,产品中除含六方氮化硼外,还含有少量立方氮化硼,生产工艺简单。  相似文献   
149.
150.
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