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   39680篇
  免费   1338篇
  国内免费   1372篇
化学   10039篇
晶体学   212篇
力学   574篇
综合类   70篇
数学   26990篇
物理学   4505篇
  2023年   386篇
  2022年   257篇
  2021年   320篇
  2020年   509篇
  2019年   950篇
  2018年   931篇
  2017年   617篇
  2016年   653篇
  2015年   716篇
  2014年   1162篇
  2013年   2393篇
  2012年   1361篇
  2011年   2553篇
  2010年   2011篇
  2009年   2707篇
  2008年   3032篇
  2007年   2906篇
  2006年   2187篇
  2005年   1590篇
  2004年   1436篇
  2003年   1268篇
  2002年   1382篇
  2001年   825篇
  2000年   798篇
  1999年   912篇
  1998年   788篇
  1997年   617篇
  1996年   765篇
  1995年   736篇
  1994年   698篇
  1993年   568篇
  1992年   491篇
  1991年   355篇
  1990年   320篇
  1989年   344篇
  1988年   243篇
  1987年   268篇
  1986年   232篇
  1985年   324篇
  1984年   261篇
  1983年   169篇
  1982年   256篇
  1981年   239篇
  1980年   177篇
  1979年   137篇
  1978年   160篇
  1977年   140篇
  1976年   110篇
  1975年   32篇
  1974年   32篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
《Mendeleev Communications》2022,32(1):126-128
3-Aryl-5-methylidene-2-thiohydantoins were constructed in one-pot reaction of aryl isothiocyanates and 3-morpholino- alanine in alkaline medium with the subsequent treatment with boiling hydrochloric acid.  相似文献   
2.
《Mendeleev Communications》2022,32(5):629-631
The Michael–Mannich cascade cyclization of cyano olefins, ethyl 4,4,4-trifluoro-3-oxobutanoate, aromatic aldehydes and ammonium acetate provides convenient stereoselective formation of ethyl 5,5-dicyano-4,6-diaryl-2-hydroxy-2-(trifluoromethyl)piperidine-3-carboxylates with fourstereogenic centers and dialkyl 4,6-diaryl-5-cyano-2-hydroxy-2-(trifluoromethyl)piperidine-3,5-dicarboxylates with five stereogenic centers. Ammonium acetate plays dual role, acting as a base and as a nitrogen source.  相似文献   
3.
Like the lower central series of a nilpotent group, filters generalize the connection between nilpotent groups and graded Lie rings. However, unlike the case with the lower central series, the associated graded Lie ring may share few features with the original group: e.g. the associated Lie ring can be trivial or arbitrarily large. We determine properties of filters such that every isomorphism between groups is induced by an isomorphism between graded Lie rings.  相似文献   
4.
5.
In this paper, let (Mn,g,dμ) be n-dimensional noncompact metric measure space which satisfies Poincaré inequality with some Ricci curvature condition. We obtain a Liouville theorem for positive weak solutions to weighted p-Lichnerowicz equation
p,fv+cvσ=0,
where c0,m>n1,1<p<m?1+(m?1)(m+3)2,σp?1 are real constants.  相似文献   
6.
The growth-fragmentation equation describes a system of growing and dividing particles, and arises in models of cell division, protein polymerisation and even telecommunications protocols. Several important questions about the equation concern the asymptotic behaviour of solutions at large times: at what rate do they converge to zero or infinity, and what does the asymptotic profile of the solutions look like? Does the rescaled solution converge to its asymptotic profile at an exponential speed? These questions have traditionally been studied using analytic techniques such as entropy methods or splitting of operators. In this work, we present a probabilistic approach: we use a Feynman–Kac formula to relate the solution of the growth-fragmentation equation to the semigroup of a Markov process, and characterise the rate of decay or growth in terms of this process. We then identify the Malthus exponent and the asymptotic profile in terms of a related Markov process, and give a spectral interpretation in terms of the growth-fragmentation operator and its dual.  相似文献   
7.
8.
9.
In the view of substrate availability, atomic efficiency and cost, directly using arenols as coupling partners in cross‐coupling, would be one of the most attractive goals. Up to date, many efforts have been made to activate the C—O bond of phenols with different strategies, for example, through in‐situ formed intermediates, through a catalytic reductive dearomatization‐condensation‐rearomatization sequence or catalytic deoxygenation. In this review, we summarized recent advances in cross‐couplings of arenols as the electrophiles via C—O activation.  相似文献   
10.
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