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   39437篇
  免费   89篇
  国内免费   133篇
系统科学   207篇
丛书文集   742篇
教育与普及   96篇
理论与方法论   222篇
现状及发展   17842篇
研究方法   1579篇
综合类   18500篇
自然研究   471篇
  2013年   202篇
  2012年   530篇
  2011年   1026篇
  2010年   234篇
  2008年   656篇
  2007年   709篇
  2006年   740篇
  2005年   738篇
  2004年   643篇
  2003年   692篇
  2002年   730篇
  2001年   1193篇
  2000年   1104篇
  1999年   733篇
  1992年   711篇
  1991年   573篇
  1990年   610篇
  1989年   616篇
  1988年   612篇
  1987年   628篇
  1986年   629篇
  1985年   770篇
  1984年   655篇
  1983年   531篇
  1982年   475篇
  1981年   484篇
  1980年   570篇
  1979年   1275篇
  1978年   1119篇
  1977年   1034篇
  1976年   798篇
  1975年   839篇
  1974年   1144篇
  1973年   980篇
  1972年   1000篇
  1971年   1260篇
  1970年   1606篇
  1969年   1203篇
  1968年   1183篇
  1967年   1208篇
  1966年   1031篇
  1965年   764篇
  1964年   229篇
  1959年   433篇
  1958年   649篇
  1957年   514篇
  1956年   438篇
  1955年   382篇
  1954年   421篇
  1948年   245篇
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
91.
92.
93.
1 Results Classic oxidants require rigorous control of the experimental conditions added with the problem of lack of selectivity. Catalysis by transition metals with environmentally safe oxidants provides synthetic routes to minimize pollution by giving environmental benign by-products. Fe (Ⅵ) is a powerful and a selective oxidant with Fe(Ⅲ) as a by-product, while hydrogen peroxide is clean with water as the only by-product. Separation of sodium or potassium ferrates requires tedious processes. Associat...  相似文献   
94.
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   
95.
96.
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   
97.
98.
Enzymatic control of cell division in micro-organisms   总被引:1,自引:0,他引:1  
NICKERSON WJ 《Nature》1948,162(4111):241-245
  相似文献   
99.
100.
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   
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