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   37145篇
  免费   701篇
  国内免费   68篇
系统科学   1207篇
丛书文集   717篇
教育与普及   80篇
理论与方法论   467篇
现状及发展   14656篇
研究方法   1384篇
综合类   19019篇
自然研究   384篇
  2018年   772篇
  2017年   788篇
  2016年   516篇
  2012年   692篇
  2011年   1786篇
  2010年   995篇
  2009年   583篇
  2008年   1061篇
  2007年   1449篇
  2006年   610篇
  2005年   642篇
  2004年   653篇
  2003年   779篇
  2002年   664篇
  2001年   979篇
  2000年   904篇
  1999年   602篇
  1992年   572篇
  1991年   466篇
  1990年   486篇
  1989年   490篇
  1988年   492篇
  1987年   502篇
  1986年   492篇
  1985年   602篇
  1984年   495篇
  1983年   404篇
  1982年   351篇
  1981年   341篇
  1980年   451篇
  1979年   975篇
  1978年   857篇
  1977年   848篇
  1976年   584篇
  1975年   633篇
  1974年   936篇
  1973年   792篇
  1972年   806篇
  1971年   1019篇
  1970年   1346篇
  1969年   1007篇
  1968年   954篇
  1967年   995篇
  1966年   831篇
  1965年   612篇
  1959年   360篇
  1958年   524篇
  1957年   447篇
  1956年   367篇
  1954年   364篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
By analyzing the observation data from Dongchuan Debris Flow Observation and Research Station and historical data from year 1965 to 1990 gotten from National Astronomical Ob-servatories/Yunnan Observatory,the responding of debris flow in Jiangjia Ravine to Solar Proton Flare is studied. The following conclusion can be drawn. Solar Proton Flare,as one of most im-portant astronomical factors,affects the activity of debris flow in Yunnan. Generally,from 1965 to1990,the more active Solar Pro-ton Flare is,the greater the probability of high frequency and large runoff of debris flow is. On the contrary,the less active Solar Pro-ton Flare is,the greater the probability of low frequency,small runoff,and low sediment transport of debris flow is.  相似文献   
43.
1 Introduction   Nanoscale science and technology have attracted greatattention since the novel properties become dominant for well-known materials as their sizes reduced to some critical dimension. These properties frequently originate in lattice distortions, structure transformations etc[1]. The variations of size and structure ofnanocrystals lead to the change of their electronic structures. Very recently, nanoscale materials with different electronic structures were utilized in fabricating nanoscale junctions[2].   ……  相似文献   
44.
This paper assesses the information content of two survey indicators for consumption developments in the near future for eight European countries in the period 1985–1998. Empirical work on this topic typically focuses on consumer confidence, the perceptions of buyers of consumption goods. This paper examines whether perceptions of sellers of consumption goods, measured by retail trade surveys, may also improve short‐term monitoring of consumption. We find that both consumer confidence and retailer confidence embody valuable information, when analysed in isolation. For France, Italy and Spain we conclude that adding retail confidence does not improve the indicator model once consumer confidence has been included. For the UK the reverse case is obtained. For the remaining four countries we show that combining consumer sentiment and retail trade confidence into a composite indicator leads to optimal results. Our results suggest that incorporating information from retail trade surveys may offer significant benefits for the analysis of short‐term prospects of consumption. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
45.
46.
The unified chaotic system contains the Lorenz system and the Chen system as two dual systems at the two extremes of its parameter spectrum. This paper presents the design of bang bang controller for unified system and multitude of numerical experiments under various control parameters. Numerical experiments meet the theoretic proof perfectly and convincingly demonstrated the controller can be effectively used for unified systems with uncertainty of the equilibrium points. The method enriches the applications of chaotic control. Foundation item: Supported by the National Natural Science Foundation of China(50209012) Biography: Deng Xiao-ming (1980-), male, Master candidate, research direction: chaos control.  相似文献   
47.
A linear regression model with random walk coefficients is extended to allow for linear restrictions between the coefficients to be satisfied at each point in time. Estimation in this model is shown to be no more involved than estimation in the standard model. It is also demonstrated how, after a slight modification to the testing problem, classical test procedures may be applied to the problem of testing for such restrictions. The performance of the Lagrange Multiplier test for a variety of different restrictions is then investigated via simulation. An empirical application involving testing for homogeneity in a random walk coefficient version of the AIDS model is given.  相似文献   
48.
DNA fingerprinting transforms the art of cell authentication.   总被引:4,自引:0,他引:4  
G N Stacey  B J Bolton  A Doyle 《Nature》1992,357(6375):261-262
The increasing diversity of new cell cultures is seriously stretching the capabilities of traditional methods of identification. DNA fingerprinting is set to play an important role in increasing confidence in the authenticity of cultures in research and industry.  相似文献   
49.
We recently reported on a linkage study within a Quarter Horse lineage segregating hyperkalaemic periodic paralysis (HYPP), an autosomal dominant condition showing potassium-induced attacks of skeletal muscle paralysis. HYPP co-segregated with the equine adult skeletal muscle sodium channel alpha subunit gene, the same gene that causes human HYPP. We now describe the Phe to Leu mutation in transmembrane domain IVS3 which courses the horse disease. This represents the first application of molecular genetics to an important horse disease, and the data will provide an opportunity for control or eradication of this condition.  相似文献   
50.
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