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   65370篇
  免费   287篇
  国内免费   559篇
系统科学   1251篇
丛书文集   571篇
教育与普及   280篇
理论与方法论   533篇
现状及发展   34715篇
研究方法   1345篇
综合类   25332篇
自然研究   2189篇
  2013年   820篇
  2012年   760篇
  2011年   2905篇
  2009年   621篇
  2008年   981篇
  2007年   1172篇
  2006年   1172篇
  2005年   1394篇
  2004年   2562篇
  2003年   2088篇
  2002年   1825篇
  2001年   1756篇
  2000年   1264篇
  1999年   1194篇
  1998年   649篇
  1997年   778篇
  1994年   698篇
  1993年   694篇
  1992年   1123篇
  1991年   907篇
  1990年   999篇
  1989年   818篇
  1988年   781篇
  1987年   809篇
  1986年   869篇
  1985年   1068篇
  1984年   878篇
  1983年   768篇
  1982年   837篇
  1981年   883篇
  1980年   983篇
  1979年   1657篇
  1978年   1457篇
  1977年   1441篇
  1976年   1244篇
  1975年   1246篇
  1974年   1277篇
  1973年   1393篇
  1972年   1503篇
  1971年   1561篇
  1970年   1869篇
  1969年   1581篇
  1968年   1483篇
  1967年   1404篇
  1966年   1212篇
  1965年   951篇
  1964年   540篇
  1959年   557篇
  1958年   829篇
  1957年   661篇
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
231.
Summary Chalcone, flavanone, flavone, and its derivatives had a certain bacteriostatic effect onSt. aureus. Flavonol has no activity and morin, a derivative of flavonol, showed only a weak inhibition of the bacterial growth.Cystein did not have any antagonistic effect.Extracts of drugs, which contain natural flavanones, flavones, isoflavones and flavonols, are more or less bacteriostatic.  相似文献   
232.
233.
Image-based gait analysis as a means of biometric identification has attracted much research attention. Most of the existing methods focus on human identification, posture analysis and movement tracking. There have been few investigations on measuring the carried load based on the carrier's gait characteristics by automatic image processing. Nevertheless, this measurement is very useful in a number of applications, such as the study of the carried load on the postural development of children and adolescence. In this paper, we investigate how to automatically estimate the carried weight from a sequence of images. We present a method to extract human gait silhouette based on an observation that humans tend to minimize the energy during motion. We compute several angles of body leaning and determine the relationship of the carried weight, the leaning angles and the centroid location according to a human kinetic study. Our weight determination method has been verified successfully by experiments.  相似文献   
234.
235.
236.
237.
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   
238.
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   
239.
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   
240.
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   
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