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   331773篇
  免费   4722篇
  国内免费   1176篇
化学   180667篇
晶体学   4454篇
力学   13615篇
综合类   16篇
数学   39414篇
物理学   99505篇
  2021年   2024篇
  2020年   2279篇
  2019年   2232篇
  2018年   2311篇
  2017年   2195篇
  2016年   4409篇
  2015年   3673篇
  2014年   4929篇
  2013年   14786篇
  2012年   11722篇
  2011年   14460篇
  2010年   8903篇
  2009年   8854篇
  2008年   13250篇
  2007年   13439篇
  2006年   13014篇
  2005年   12287篇
  2004年   11249篇
  2003年   9652篇
  2002年   9217篇
  2001年   10477篇
  2000年   8082篇
  1999年   6319篇
  1998年   5063篇
  1997年   4923篇
  1996年   5046篇
  1995年   4605篇
  1994年   4353篇
  1993年   4192篇
  1992年   4687篇
  1991年   4468篇
  1990年   4167篇
  1989年   3972篇
  1988年   4237篇
  1987年   3901篇
  1986年   3798篇
  1985年   5454篇
  1984年   5514篇
  1983年   4480篇
  1982年   4871篇
  1981年   4894篇
  1980年   4650篇
  1979年   4745篇
  1978年   4771篇
  1977年   4731篇
  1976年   4674篇
  1975年   4567篇
  1974年   4403篇
  1973年   4562篇
  1972年   2558篇
排序方式: 共有10000条查询结果,搜索用时 640 毫秒
21.
We summarize the results of our recent work on Bäcklund transformations (BTs), particularly focusing on the relation between BTs and infinitesimal symmetries. We present a BT for an associated Degasperis–Procesi (aDP) equation and its superposition principle and investigate the solutions generated by applying this BT. Following our general methodology, we use the superposition principle of the BT to generate the infinitesimal symmetries of the aDP equation.  相似文献   
22.
23.
24.
The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20–30 kcal mol−1 and a polarity (μ) between 7–20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation.  相似文献   
25.
26.
27.
28.
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   
29.
30.
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