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   528613篇
  免费   4156篇
  国内免费   1281篇
化学   269045篇
晶体学   7645篇
力学   25990篇
综合类   17篇
数学   67779篇
物理学   163574篇
  2021年   5035篇
  2020年   5477篇
  2019年   6221篇
  2018年   8413篇
  2017年   8560篇
  2016年   11625篇
  2015年   6176篇
  2014年   10437篇
  2013年   23486篇
  2012年   18335篇
  2011年   21788篇
  2010年   16419篇
  2009年   16279篇
  2008年   20656篇
  2007年   20508篇
  2006年   18612篇
  2005年   16620篇
  2004年   15469篇
  2003年   13955篇
  2002年   13847篇
  2001年   14728篇
  2000年   11343篇
  1999年   8752篇
  1998年   7600篇
  1997年   7470篇
  1996年   6928篇
  1995年   6257篇
  1994年   6264篇
  1993年   6017篇
  1992年   6375篇
  1991年   6865篇
  1990年   6577篇
  1989年   6486篇
  1988年   6321篇
  1987年   6132篇
  1986年   5871篇
  1985年   7408篇
  1984年   7769篇
  1983年   6532篇
  1982年   6817篇
  1981年   6326篇
  1980年   6003篇
  1979年   6504篇
  1978年   6741篇
  1977年   6629篇
  1976年   6586篇
  1975年   6302篇
  1974年   6129篇
  1973年   6424篇
  1972年   4706篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
For a graph G and a positive integer m, G(m) is the graph obtained from G by replacing every vertex by an independent set of size m and every edge by m2 edges joining all possible new pairs of ends. If G triangulates a surface, then it is easy to see from Euler's formula that G(m) can, in principle, triangulate a surface. For m prime and at least 7, it has previously been shown that in fact G(m) does triangulate a surface, and in fact does so as a “covering with folds” of the original triangulation. For m = 5, this would be a consequence of Tutte's 5‐Flow Conjecture. In this work, we investigate the case m = 2 and describe simple classes of triangulations G for which G(2) does have a triangulation that covers G “with folds,” as well as providing a simple infinite class of triangulations G of the sphere for which G(2) does not triangulate any surface. © 2003 Wiley Periodicals, Inc. J Graph Theory 43: 79–92, 2003  相似文献   
82.
Crystals of Saccharomyces cerevisiae inorganic pyrophosphatase suitable for X-ray diffraction study were grown by cocrystallization of the enzyme with cobalt chloride and imidodiphosphate. Saccharomyces cerevisiae is a metal-dependent enzyme which catalyzes hydrolysis of inorganic pyrophosphate to orthophosphate. The three-dimensional structure of this enzyme was solved by the molecular-replacement method and refined at 1.8 Å resolution to an R factor of 19.5%. Cobalt and phosphate ions were revealed in the active centers of both identical subunits (A and B) of the pyrophosphatase molecule. In subunit B, a water molecule was found between two cobalt ions. It is believed that this water molecule acts as an attacking nucleophile in the enzymatic cleavage of the pyrophosphate bond. It was demonstrated that cobalt ions and a phosphate group occupy only part of the potential binding sites (two chemically identical and crystallographically independent subunits have different binding sites). The arrangement of ligands and the structure of the nucleophile-binding site are discussed in relation to the mechanism of action of the enzyme and the nature of the metal activator.  相似文献   
83.
84.
For three‐dimensional flows with one inhomogeneous spatial coordinate and two periodic directions, the Karhunen–Loeve procedure is typically formulated as a spatial eigenvalue problem. This is normally referred to as the direct method (DM). Here we derive an equivalent formulation in which the eigenvalue problem is formulated in the temporal coordinate. It is shown that this so‐called method of snapshots (MOS) has some numerical advantages when compared to the DM. In particular, the MOS can be formulated purely as a matrix composed of scalars, thus avoiding the need to construct a matrix of matrices as in the DM. In addition, the MOS avoids the need for so‐called weight functions, which emerge in the DM as a result of the non‐uniform grid typically employed in the inhomogeneous direction. The avoidance of such weight functions, which may exhibit singular behaviour, guarantees satisfaction of the boundary conditions. The MOS is applied to data sets recently obtained from the direct simulation of turbulence in a channel in which viscoelasticity is imparted to the fluid using a Giesekus model. The analysis reveals a steep drop in the dimensionality of the turbulence as viscoelasticity is increased. This is consistent with the results that have been obtained with other viscoelastic models, thus revealing an essential generic feature of polymer‐induced drag reduced turbulent flows. Published in 2006 by John Wiley & Sons, Ltd.  相似文献   
85.
86.
The dependence of the energy characteristics on an adiabatically slowly increasing external force is determined analytically for an anharmonic oscillator. The analytical results are confirmed by a numerical calculation. The nature of the force dependences of the energy characteristics are determined and discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 153–157 (January 1997)  相似文献   
87.
Mass distributions of fragments in the low-energy fission of nuclei from 187Ir to 213At have been analysed. This analysis has shown that shell effects in symmetric-mode fragment mass yields from the fission of pre-actinide nuclei could be described if one assumes the existence of two strongly deformed neutron shells in the arising fragments with neutron numbers N1 ≈ 52 and N2 ≈ 68. A new method has been proposed for quantitatively describing the mass distributions of the symmetric fission mode for pre-actinides with A ≈ 180–220.  相似文献   
88.
89.
It is shown that the exact solution of the problem of convective heat exchange in a low-Prandtl-number fluid flow (Pr ? 1) can be obtained for a two-dimensional body of arbitrary shape provided that the conformal mapping relating the body contour with a circle is known. By way of example, the Joukowsky airfoil is considered.  相似文献   
90.
A class of Hamiltonian dynamic systems integrated by the variable separation method is considered. The integration for this class is the inversion of an Abel mapping on hyperelliptic curves. We prove that the derivative of the Abel mapping is the Stäckel matrix, which determines a diagonal Riemannian metric and curvilinear orthogonal coordinate systems in a flat space. Lax representations with the spectral parameter are constructed. The corresponding classicalr-matrices are dynamic. It is shown how the class of pointwise canonical transformations can be naturally generalized using the Abel integral reduction theory.  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