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   52687篇
  免费   365篇
  国内免费   612篇
系统科学   1270篇
丛书文集   192篇
教育与普及   283篇
理论与方法论   518篇
现状及发展   31342篇
研究方法   607篇
综合类   17258篇
自然研究   2194篇
  2013年   856篇
  2012年   595篇
  2011年   2613篇
  2010年   493篇
  2009年   641篇
  2008年   800篇
  2007年   895篇
  2006年   963篇
  2005年   1123篇
  2004年   2343篇
  2003年   1856篇
  2002年   1556篇
  2001年   1304篇
  2000年   845篇
  1999年   922篇
  1998年   644篇
  1997年   778篇
  1996年   534篇
  1994年   688篇
  1993年   689篇
  1992年   809篇
  1991年   711篇
  1990年   764篇
  1989年   587篇
  1988年   559篇
  1987年   570篇
  1986年   642篇
  1985年   794篇
  1984年   722篇
  1983年   610篇
  1982年   758篇
  1981年   798篇
  1980年   873篇
  1979年   1292篇
  1978年   1156篇
  1977年   1169篇
  1976年   1018篇
  1975年   998篇
  1974年   858篇
  1973年   1102篇
  1972年   1192篇
  1971年   1162篇
  1970年   1221篇
  1969年   1138篇
  1968年   1098篇
  1967年   972篇
  1966年   807篇
  1965年   667篇
  1964年   487篇
  1958年   451篇
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
201.
1 Results The photosynthetic bacterial reaction center (RC) is a membrane protein complex.The RC is composed of three protein subunits and redox components such as bacteriochlorophylls, bacteriopheophytins,and quinones.The RC performs the photochemical electron transfer from the bacteriochlorophyll dimer through a series of electron donor and acceptor molecules to a secondary quinone,QB.QB accepts electrons from a primary quinone,QA,in two sequential electron transfer reactions.The second electron trans...  相似文献   
202.
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   
203.
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   
204.
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   
205.
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   
206.
207.
208.
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   
209.
210.
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