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   65089篇
  免费   281篇
  国内免费   554篇
系统科学   1259篇
丛书文集   445篇
教育与普及   295篇
理论与方法论   545篇
现状及发展   34688篇
研究方法   1266篇
综合类   25231篇
自然研究   2195篇
  2013年   810篇
  2012年   707篇
  2011年   2808篇
  2009年   633篇
  2008年   949篇
  2007年   1071篇
  2006年   1167篇
  2005年   1303篇
  2004年   2516篇
  2003年   2082篇
  2002年   1753篇
  2001年   1778篇
  2000年   1304篇
  1999年   1173篇
  1998年   652篇
  1997年   779篇
  1996年   537篇
  1994年   693篇
  1993年   695篇
  1992年   1097篇
  1991年   913篇
  1990年   1030篇
  1989年   818篇
  1988年   760篇
  1987年   818篇
  1986年   890篇
  1985年   1075篇
  1984年   889篇
  1983年   775篇
  1982年   881篇
  1981年   900篇
  1980年   981篇
  1979年   1720篇
  1978年   1506篇
  1977年   1445篇
  1976年   1289篇
  1975年   1279篇
  1974年   1306篇
  1973年   1435篇
  1972年   1459篇
  1971年   1582篇
  1970年   1792篇
  1969年   1557篇
  1968年   1524篇
  1967年   1400篇
  1966年   1201篇
  1965年   973篇
  1959年   547篇
  1958年   785篇
  1957年   624篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
231.
1 Results The entrapment of biopolymers into silica by the sol-gel technique meets with incompatibility of inorganic and bioorganic components. The aim was to develop a compatible procedure biomimicking the biomineralization processes in the living nature. A suggested solution in Ref.[1-2] for the biopolymer entrapment into silica matrix is based on a novel silica precursor. The developed approach is distinguished from the common technique for fabrication of hybrid biopolymer-silica nanocomposite materi...  相似文献   
232.
It is well known that considering a non-Euclidean Minkowski metric in Multidimensional Scaling, either for the distance model or for the loss function, increases the computational problem of local minima considerably. In this paper, we propose an algorithm in which both the loss function and the composition rule can be considered in any Minkowski metric, using a multivariate randomly alternating Simulated Annealing procedure with permutation and translation phases. The algorithm has been implemented in Fortran and tested over classical and simulated data matrices with sizes up to 200 objects. A study has been carried out with some of the common loss functions to determine the most suitable values for the main parameters. The experimental results confirm the theoretical expectation that Simulated Annealing is a suitable strategy to deal by itself with the optimization problems in Multidimensional Scaling, in particular for City-Block, Euclidean and Infinity metrics.  相似文献   
233.
The aim of this paper is to compare the forecasting performance of competing threshold models, in order to capture the asymmetric effect in the volatility. We focus on examining the relative out‐of‐sample forecasting ability of the SETAR‐Threshold GARCH (SETAR‐TGARCH) and the SETAR‐Threshold Stochastic Volatility (SETAR‐THSV) models compared to the GARCH model and Stochastic Volatility (SV) model. However, the main problem in evaluating the predictive ability of volatility models is that the ‘true’ underlying volatility process is not observable and thus a proxy must be defined for the unobservable volatility. For the class of nonlinear state space models (SETAR‐THSV and SV), a modified version of the SIR algorithm has been used to estimate the unknown parameters. The forecasting performance of competing models has been compared for two return time series: IBEX 35 and S&P 500. We explore whether the increase in the complexity of the model implies that its forecasting ability improves. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
234.
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   
235.
In this paper, we introduce a new class of generalized convex function, namely, a-pseudounivex function, by combining the concepts of pseudo-univex and α-invex functions. Further, we establish some relationships between vector variational-like inequality problems and vector optimization problems under the assumptions of α-pseudo-univex functions. Results obtained in this paper present a refinement and improvement of previously known results.  相似文献   
236.
237.
238.
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   
239.
240.
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   
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