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   8824篇
  免费   1709篇
  国内免费   1071篇
化学   6651篇
晶体学   94篇
力学   430篇
综合类   96篇
数学   878篇
物理学   3455篇
  2024年   24篇
  2023年   188篇
  2022年   294篇
  2021年   349篇
  2020年   428篇
  2019年   475篇
  2018年   372篇
  2017年   338篇
  2016年   494篇
  2015年   479篇
  2014年   576篇
  2013年   708篇
  2012年   884篇
  2011年   837篇
  2010年   613篇
  2009年   525篇
  2008年   598篇
  2007年   556篇
  2006年   458篇
  2005年   346篇
  2004年   248篇
  2003年   237篇
  2002年   267篇
  2001年   249篇
  2000年   144篇
  1999年   188篇
  1998年   126篇
  1997年   86篇
  1996年   80篇
  1995年   55篇
  1994年   57篇
  1993年   59篇
  1992年   48篇
  1991年   54篇
  1990年   40篇
  1989年   35篇
  1988年   22篇
  1987年   18篇
  1986年   13篇
  1985年   9篇
  1984年   8篇
  1983年   8篇
  1982年   3篇
  1981年   2篇
  1980年   2篇
  1979年   1篇
  1959年   2篇
  1936年   1篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
Let be a locally strongly convex hypersurface, given by the graph of a convex function xn+1=f(x1,…,xn) defined in a convex domain ΩRn. M is called a α-extremal hypersurface, if f is a solution of
  相似文献   
992.
The cubic boron nitride (cBN) is a kind of artificial electro-optic (EO) crystal, and we have not found any relative reports so far. Because the artificial synthetic cBN wafers are very small and hard, the wafers cannot be cut into rectangular slabs. The polarizer-sample-λ/4 retardation plate (compensator)-analyzer (PSCA) transverse EO modulator has to be adjusted to the minute irregular octahedron of cBN wafers. When the applied voltage is along [1 1 1] direction of the wafer, due to refraction, the angle between the incident beam direction and the (1 1 1) plane (top or bottom plane) of the wafer should be 25.4°, and the angle between the polarization direction of the polarizer and the plane of incidence should be 50.8° by calculation, respectively. The half-wave voltage of the cBN sample was obtained for the first time, by means of detection of the output optic signals from the modulator with and without an applied electric field on the sample, respectively. Furthermore, the linear EO coefficient was obtained, . The analysis of the experimental resulting error was carried out.  相似文献   
993.
Jia  Xiaoxi  Kanzow  Christian  Mehlitz  Patrick  Wachsmuth  Gerd 《Mathematical Programming》2023,199(1-2):1365-1415

This paper is devoted to the theoretical and numerical investigation of an augmented Lagrangian method for the solution of optimization problems with geometric constraints. Specifically, we study situations where parts of the constraints are nonconvex and possibly complicated, but allow for a fast computation of projections onto this nonconvex set. Typical problem classes which satisfy this requirement are optimization problems with disjunctive constraints (like complementarity or cardinality constraints) as well as optimization problems over sets of matrices which have to satisfy additional rank constraints. The key idea behind our method is to keep these complicated constraints explicitly in the constraints and to penalize only the remaining constraints by an augmented Lagrangian function. The resulting subproblems are then solved with the aid of a problem-tailored nonmonotone projected gradient method. The corresponding convergence theory allows for an inexact solution of these subproblems. Nevertheless, the overall algorithm computes so-called Mordukhovich-stationary points of the original problem under a mild asymptotic regularity condition, which is generally weaker than most of the respective available problem-tailored constraint qualifications. Extensive numerical experiments addressing complementarity- and cardinality-constrained optimization problems as well as a semidefinite reformulation of MAXCUT problems visualize the power of our approach.

  相似文献   
994.
It is well known that the classical Ascoli-Arzelà theorem is powerful technique to give a necessary and sufficient condition for investigating the relative compactness of a family of abstract continuous functions, while it is limited to finite compact interval. In this paper, we shall generalize the Ascoli-Arzelà theorem on an infinite interval. As its application, we investigate an initial value problem for fractional evolution equations on infinite interval in the sense of Hilfer type, which is a generalization of both Riemann-Liuoville and Caputo fractional derivatives. Our methods are based on the Hausdorff theorem, classical/generalized Ascoli-Arzelà theorem, Schauder fixed point theorem, Wright function, and Kuratowski measure of noncompactness. We obtain the existence of mild solutions on an infinite interval when the semigroup is compact as well as noncompact.  相似文献   
995.
In this paper, we present a two-grid finite element method for the Allen-Cahn equation with the logarithmic potential. This method consists of two steps. In the first step, based on a fully implicit finite element method, the Allen-Cahn equation is solved on a coarse grid with mesh size H. In the second step, a linearized system whose nonlinear term is replaced by the value of the first step is solved on a fine grid with mesh size h. We give the energy stabilities of the traditional finite element method and the two-grid finite element method. The optimal convergence order of the two-grid finite element method in H1 norm is achieved when the mesh sizes satisfy h = O(H2). Numerical examples are given to demonstrate the validity of the proposed scheme. The results show that the two-grid method can save the CPU time while keeping the same convergence rate.  相似文献   
996.
B型套筒修复作为油气管道修复的主要方式之一,在其修复的焊接过程中,该结构的环形角焊缝内部常常出现以气孔、夹渣为代表的孔型缺陷。这类缺陷由于角焊缝特殊的结构,在利用相控阵超声无损检测技术对其检测时,难以对其进行定量分析。通过数值模拟和实验,发现通过相控阵超声扇扫得到缺陷回波信号峰值与孔型缺陷直径呈正相关,为孔型缺陷的定量分析提供了一种可靠的手段。  相似文献   
997.
Photonic graphene, possesses a honeycomb-like geometric structure, provides a superior platform for simulating photonic bandgap, Dirac physics, and topological photonics. Here, the photonic graphene with reconfigurable geometric structures is demonstrated in a 5S1/2 − 5P3/2 − 5D5/2 cascade-type 85Rb atomic ensembles. A strong hexagonal-coupling field, formed by the interference of three identical coupling beams, is responsible for optically inducing photonic graphene in atomic vapor. The incident weak probe beam experiences discrete diffraction, and the observed pattern at the output plane of vapor cell exhibits a clear hexagonal intensity distribution. The complete photonic graphene geometries from transversely stretched to longitudinally stretched are conveniently constructed by varying the spatial arrangement of three coupling beams, and the corresponding diffraction patterns are implemented theoretically and experimentally to map these distorted geometric structures. Moreover, the distribution of lattice sites intensity in photonic graphene is further dynamically adjusted by two-photon detuning and the coupling beams power. This work paves the way for further investigation of light transport and graphene dynamics.  相似文献   
998.
机动方舱核爆炸冲击波动力响应和安定性分析   总被引:1,自引:0,他引:1  
本文以核爆炸当量、高度、距离为参数,采用等超压圆给出了普通方舱和防核加固方舱结构在核冲击波下的安全域。研究了限制舱体倾斜的压力条件。本文还以冲击波压力的时间函数为激励,求解了方舱的动力响应。从试验结果推论和计算模拟两方面,分析了方舱承受冲击波的能力。  相似文献   
999.
在Reddy型高阶壳体理论的基础上,采用沿壳体厚度方向的剪切应变呈抛物线分布并且能够满足在壳体的上下表面为零的假设,发展出了一种适合于对变厚度壳体进行非线性分析的方法。该方法利用Ritz原理得到问题的控制方程。通过对一种典型的变厚度壳体结构(子午线轮胎)的结构分析,该方法的计算结果与商用有限元软件的三维分析结果能够很好的吻合。表明了该方法的适用性和有效性。  相似文献   
1000.
弹/粘塑性柱体扭转问题的函数Laplace变换解   总被引:3,自引:1,他引:3  
贾乃文 《力学学报》1995,27(4):434-439
在应力Laplace变换分析粘塑性轴对称问题基础上,对弹性-弹/粘塑性圆柱体扭转全过程进行分析,根据柱体扭转的应力分布,构造应力函数与位移函数,并对函数进行Laplace变换。相应求出圆柱体、空心圆柱体的Laplace变换解,以及圆柱体的弹性-粘塑性交界线值  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