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   73882篇
  免费   362篇
  国内免费   782篇
系统科学   1459篇
丛书文集   900篇
教育与普及   324篇
理论与方法论   559篇
现状及发展   36829篇
研究方法   1588篇
综合类   31173篇
自然研究   2194篇
  2013年   1032篇
  2012年   1080篇
  2011年   3232篇
  2010年   775篇
  2009年   894篇
  2008年   1328篇
  2007年   1566篇
  2006年   1608篇
  2005年   1663篇
  2004年   2651篇
  2003年   2200篇
  2002年   1923篇
  2001年   2010篇
  2000年   1579篇
  1999年   1350篇
  1998年   681篇
  1997年   792篇
  1994年   691篇
  1993年   695篇
  1992年   1236篇
  1991年   1020篇
  1990年   1111篇
  1989年   936篇
  1988年   877篇
  1987年   972篇
  1986年   959篇
  1985年   1157篇
  1984年   982篇
  1983年   810篇
  1982年   907篇
  1981年   933篇
  1980年   1078篇
  1979年   1852篇
  1978年   1624篇
  1977年   1615篇
  1976年   1381篇
  1975年   1408篇
  1974年   1546篇
  1973年   1615篇
  1972年   1688篇
  1971年   1774篇
  1970年   2036篇
  1969年   1749篇
  1968年   1698篇
  1967年   1612篇
  1966年   1359篇
  1965年   1046篇
  1959年   613篇
  1958年   881篇
  1957年   680篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Surface Preparation is very important in adhesive b on ding of spray coatings to the surface of a work piece. The common practice is gr it-blasting of the surface before subjecting it to the spray coating process. In this study, grit-blasting of an AISI 4130 steel (of different heat treatmen ts) with Al 2O 3 particles was studied. Various grit-blasting parameters such as blasting particle size, the distance between blasting nozzle and the work pi ece (25, 30 and 40 cm.), blasting pressure (3,4,5,6 and ...  相似文献   
92.
A detailed karyotypic analysis of two Egyptian species of bats,Taphozous perforatus andTaphozous nudiventris, was made on the basis of conventional data and G-band patterns. No detectable karyotypic difference was found between the two species (2n=42, F.N. 64). By comparison of G-band patterns, some differences between the two species were seen in the corresponding autosomal pairs. These results are reported for the first time in Egypt.  相似文献   
93.
94.
Recent years have seen an increasing cross-fertilization between the fields of decision analysis and forecasting. Decision-analytic models often require forecasts as inputs, and aspects of the Bayesian decision-theoretic framework underlying decision analysis have proved useful to forecasting, particularly in contexts where subjective judgemental inputs are required. This paper describes the use of decision tree analysis for forecasting and illustrates its use for corporate divisional forecasting and planning. A specialized decision-analytic technique, acts as events, is also described and illustrated to forecast a new product's earnings. Conclusions are drawn about the applicability of decision analysis for forecasting.  相似文献   
95.
Summary Indoxyl derivatives were detected as minor products among the urinary metabolites of two trial drugs, a benzodiazepine (GP 55 129) and a benzophenone (CGP 11 952). Their structures were elucidated by NMR and mass spectroscopy. Presumably, metabolites containing potential aldehyde functions react spontaneously with endogenous indoxyl. Such derivatives have not hitherto been encountered in drug metabolism.  相似文献   
96.
本文提出了精测安培表内阻的新方法,并将其与其它测量方法进行了比较。  相似文献   
97.
本文介绍了在水电工程规划设计时,如何利用三次自然样条插值函数在有限个方案的基础上自动生成无限多个方案,然后采用多目标综合决策模型在其中选择最优方案的一种方法.文中还给出了程序流程,并以某水电站正常蓄水位选择作为实例,对方法进行了验证,结果与专家决策完全吻合.  相似文献   
98.
The gas-liquid-liquid reactive extraction system for preparing hydrogen peroxide via anthraquinone was investigated. The oxidation reaction of hydrogenated working solution was combined with the extraction of hydrogen peroxide from working solution in a sieve plate column. The reaction of 2-ethylanthrahydroquionone with oxygen and the liquid-liquid extraction of hydrogen peroxide take place simultaneously. The oxygen was introduced with hydrogenated working solution through a nozzle in the bottom of the column, which worked as agitated air as well as oxidation reagent. The results showed the oxidation and extraction do not hamper each other, on the contrary, the presence of oxidation gas in the column can promote the transfer of hydrogen peroxide from organic phase to aqueous phase, thus the reaction efficiency and extraction efficiency increased with increasing gas superficial velocity. Furthermore, the oxidation efficiency is almost 100% and the extraction efficiency is higher than 90% in this process.  相似文献   
99.
100.
提出了一种基于径向网络的增量学习算法 ,在对新样本进行学习时 ,通过恢复受干扰样本的方式来尽可能保持已有的知识不变 .模拟实验证明了本算法的正确性和有效性 .  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