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   467177篇
  免费   4550篇
  国内免费   1295篇
化学   235208篇
晶体学   7111篇
力学   23623篇
综合类   17篇
数学   56811篇
物理学   150252篇
  2021年   4275篇
  2020年   4561篇
  2019年   5301篇
  2018年   7404篇
  2017年   7590篇
  2016年   10332篇
  2015年   5666篇
  2014年   9732篇
  2013年   21704篇
  2012年   16844篇
  2011年   20128篇
  2010年   15228篇
  2009年   15315篇
  2008年   18410篇
  2007年   18278篇
  2006年   16652篇
  2005年   14571篇
  2004年   13649篇
  2003年   12175篇
  2002年   12194篇
  2001年   14156篇
  2000年   10484篇
  1999年   8095篇
  1998年   6825篇
  1997年   6732篇
  1996年   6229篇
  1995年   5456篇
  1994年   5503篇
  1993年   5282篇
  1992年   5897篇
  1991年   6190篇
  1990年   5837篇
  1989年   5854篇
  1988年   5559篇
  1987年   5692篇
  1986年   5349篇
  1985年   6794篇
  1984年   6954篇
  1983年   5773篇
  1982年   5807篇
  1981年   5512篇
  1980年   5263篇
  1979年   5836篇
  1978年   5774篇
  1977年   5980篇
  1976年   5962篇
  1975年   5611篇
  1974年   5509篇
  1973年   5621篇
  1972年   4115篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Malmquist indexes can be used to estimate technical change over time. The index generally represents the average effect of technical change on all outputs in a multi-output firm. However, changes in technology do not necessarily affect all outputs involved in the analysis equally. In this paper, a method for estimating the effect of technical change on each of the different outputs is proposed. This estimation is done by a modification of quasi-Malmquist indexes. An artificial data set has been used to validate the method proposed in this paper.  相似文献   
992.
The essential self-adjointness on a natural domain of the sharp-time Wick powers of the relativistic free field in two space-time dimension is proven. Other results on Wick powers are reviewed and discussed.  相似文献   
993.
We construct the polynomial quantization on the space G/H where G=SL(n,R),H=GL(n–1,R). It is a variant of quantization in the spirit of Berezin. In our case covariant and contravariant symbols are polynomials on G/H. We introduce a multiplication of covariant symbols, establish the correspondence principle, study transformations of symbols (the Berezin transform) and of operators. We write a full asymptotic decomposition of the Berezin transform.  相似文献   
994.
995.
For the Boussinesq approximation of the equations of coupled heat and fluid flow in a porous medium we show that the corresponding system of partial differential equations possesses a global attractor. We give lower and upper bounds of the Hausdorff dimension of the attractor depending on a physical parameter of the system, namely the Rayleigh number of the flow. Numerical experiments confirm the theoretical findings and raise new questions on the structure of the solutions of the system. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
996.
997.
An overview of the use of non-protein amino acids in the design of conformationally well-defined peptides, based on work from the author’s laboratory, is discussed. The crystal structures of several designed oligopeptides illustrate the useα-aminoisobutyric acid (Aib) in the construction of helices, D-amino acids in the design of helix termination segments andDPro-Xxx segments for nucleating ofβ-hairpin structures.β- andγ-amino acid residues have been used to expand the range of designed polypeptide structures. Dedicated to Professor C N R Rao on his 70th birthday  相似文献   
998.
Aldehydes undergo smooth conjugate addition to α,β-unsaturated ketones in the presence of 5-(2-hydroxyethyl)-1,3-thiazolium halides and DBU adsorbed onto the surface of basic alumina under microwave irradiation and solvent-free conditions to afford 1,4-diketones in enhanced yields and reduced reaction times compared to conventional methods.  相似文献   
999.
Addition of bromotrichloromethane and tetrachloromethane to alkenes in the presence of palladium(II) complexes begins with single-electron transfer from the palladium atom to polyhalomethane molecule, followed by fragmentation of the radical anion thus formed and generation of trichloromethyl radical. Halogen transfer to a carbon-centered radical arising from addition of trichloromethyl radical at the double bond occurs both from the polyhalomethane molecule and from halide palladium complex.  相似文献   
1000.
A mixed phosphonium-iodonium ylide, phenyliodoniumethoxycarbonylmethylenetriphenylphosphorane borofluoride, was synthesized. Its structure was established by means of X-ray diffraction analysis. Temperature dependence of 1H, 13C, and 31P spectra of the ylide synthesized was investigated. A dynamic equilibrium between Z and E-isomers was observed.  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