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   516586篇
  免费   7238篇
  国内免费   1848篇
化学   281522篇
晶体学   7336篇
力学   21077篇
综合类   9篇
数学   63950篇
物理学   151778篇
  2020年   3787篇
  2019年   3893篇
  2018年   4437篇
  2017年   4280篇
  2016年   7764篇
  2015年   5865篇
  2014年   8196篇
  2013年   24080篇
  2012年   17736篇
  2011年   22117篇
  2010年   13948篇
  2009年   13735篇
  2008年   20055篇
  2007年   20317篇
  2006年   19198篇
  2005年   17645篇
  2004年   15691篇
  2003年   13903篇
  2002年   13563篇
  2001年   15021篇
  2000年   11637篇
  1999年   9260篇
  1998年   7613篇
  1997年   7435篇
  1996年   7388篇
  1995年   6799篇
  1994年   6517篇
  1993年   6463篇
  1992年   7035篇
  1991年   6866篇
  1990年   6478篇
  1989年   6276篇
  1988年   6501篇
  1987年   6179篇
  1986年   5949篇
  1985年   8469篇
  1984年   8650篇
  1983年   7109篇
  1982年   7718篇
  1981年   7632篇
  1980年   7372篇
  1979年   7428篇
  1978年   7687篇
  1977年   7513篇
  1976年   7558篇
  1975年   7133篇
  1974年   7105篇
  1973年   7461篇
  1972年   4468篇
  1971年   3389篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
53.
54.
Profiling engineered data with robust mining methods continues attracting attention in knowledge engineering systems. The purpose of this article is to propose a simple technique that deals with non-linear multi-factorial multi-characteristic screening suitable for knowledge discovery studies. The method is designed to proactively seek and quantify significant information content in engineered mini-datasets. This is achieved by deploying replicated fractional-factorial sampling schemes. Compiled multi-response data are converted to a single master-response effectuated by a series of distribution-free transformations and multi-compressed data fusions. The resulting amalgamated master response is deciphered by non-linear multi-factorial stealth stochastics intended for saturated schemes. The stealth properties of our method target processing datasets which might be overwhelmed by a lack of knowledge about the nature of reference distributions at play. Stealth features are triggered to overcome restrictions regarding the data normality conformance, the effect sparsity assumption and the inherent collapse of the ‘unexplainable error’ connotation in saturated arrays. The technique is showcased by profiling four ordinary controlling factors that influence webpage content performance by collecting data from a commercial browser monitoring service on a large scale web host. The examined effects are: (1) the number of Cascading Style Sheets files, (2) the number of JavaScript files, (3) the number of Image files, and (4) the Domain Name System Aliasing. The webpage performance level was screened against three popular characteristics: (1) the time to first visual, (2) the total loading time, and (3) the customer satisfaction. Our robust multi-response data mining technique is elucidated for a ten-replicate run study dictated by an L9(34) orthogonal array scheme where any uncontrolled noise embedded contribution has not been necessarily excluded.  相似文献   
55.
56.
The results of investigation of optical image detectors designed for the largest problem, near-VUV, range of the spectrum are presented. The possibility of using a dual-stage image detection system to appreciably lower the sensitivity threshold and make computer data processing feasible is considered. The integration of a UV module into a wideband image detector is studied.  相似文献   
57.
58.
A tertiary hydroxy group α to a carboxyl moiety comprises a key structural motif in many bioactive substances. With the herein presented metal‐free rearrangement of imides triggered by hypervalent λ3‐iodane, an easy and selective way to gain access to such a compound class, namely α,α‐disubstituted‐α‐hydroxy carboxylamides, was established. Their additional methylene bromide side chain constitutes a useful handle for rapid diversification, as demonstrated by a series of further functionalizations. Moreover, the in situ formation of an iodine(III) species under the reaction conditions was proven. Our findings clearly corroborate that hypervalent λ3‐benziodoxolones are involved in these organocatalytic reactions.  相似文献   
59.
60.
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