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   99697篇
  免费   6129篇
  国内免费   15118篇
化学   80596篇
晶体学   1394篇
力学   2804篇
综合类   965篇
数学   14321篇
物理学   20864篇
  2024年   94篇
  2023年   740篇
  2022年   1438篇
  2021年   2074篇
  2020年   2513篇
  2019年   2396篇
  2018年   2115篇
  2017年   2972篇
  2016年   3197篇
  2015年   2640篇
  2014年   3783篇
  2013年   7495篇
  2012年   7038篇
  2011年   5548篇
  2010年   4714篇
  2009年   6424篇
  2008年   6687篇
  2007年   6938篇
  2006年   6332篇
  2005年   5417篇
  2004年   5027篇
  2003年   4226篇
  2002年   5396篇
  2001年   3215篇
  2000年   3003篇
  1999年   2812篇
  1998年   2467篇
  1997年   1975篇
  1996年   1698篇
  1995年   1645篇
  1994年   1463篇
  1993年   1187篇
  1992年   1193篇
  1991年   804篇
  1990年   669篇
  1989年   613篇
  1988年   452篇
  1987年   353篇
  1986年   320篇
  1985年   250篇
  1984年   258篇
  1983年   139篇
  1982年   213篇
  1981年   169篇
  1980年   187篇
  1979年   167篇
  1978年   145篇
  1977年   92篇
  1976年   74篇
  1973年   43篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
尚小明  汤国庆  张桂兰  陈文驹 《光学学报》1996,16(10):1371-1378
报道了以N2激光为泵浦光源,获得激发态分子内质子转移(ESIPT)分子HBO的激光输出现象。其激光转换效率约为17%,调谐范围为495~540nm,最强的输出波长在510nm。以HBO的激发态分子内质子转移的光物理和光化学过程为基础,建立了HBO产生激光的动态模型,数值模拟了在宽带和窄带情况下激光输出的光谱特性和时间特性,理论计算值与实验观测值很好相符,同时证实了激发态分子内质子转移分子的激光脉冲宽度依赖于激发态分子内质子转移分子的酮式异构体的基态S′0的寿命  相似文献   
992.
Summary. The dimensional reduction method for solving boundary value problems of Helmholtz's equation in domain by replacing them with systems of equations in dimensional space are investigated. It is proved that the existence and uniqueness for the exact solution and the dimensionally reduced solution of the boundary value problem if the input data on the faces are in some class of functions. In addition, the difference between and in is estimated as and are fixed. Finally, some numerical experiments in a domain are given in order to compare theretical results. Received April 2, 1996 / Revised version received July 30, 1990  相似文献   
993.
994.
An evolutionary method for optimization of plate buckling resistance   总被引:9,自引:0,他引:9  
Optimization of plate buckling resistance is very complicated, because the in-plane stress resultants in the prebuckled state of a plate are functions of thickness distribution. This paper discusses the problem of finding the optimum thickness distribution of isotropic plate structures, with a given volume and layout, that would maximise the buckling load. A simple numerical method using the finite-element analysis is presented to obtain the optimum thickness distribution. Optimum designs of compression-loaded rectangular plates with different boundary conditions and plate aspect ratios are obtained by using the proposed method. Optimum designs from earlier studies and the methods of buckling analysis used to attain these results are discussed and compared with the designs from the proposed method. This paper also examines the reliability of the optimality criterion generally used for plate buckling optimization, which is based on the uniform strain energy density.  相似文献   
995.
We study the large-time behavior and rate of convergence to the invariant measures of the processes dX (t)=b(X) (t)) dt + (X (t)) dB(t). A crucial constant appears naturally in our study. Heuristically, when the time is of the order exp( – )/2 , the transition density has a good lower bound and when the process has run for about exp( – )/2, it is very close to the invariant measure. LetL =(2/2) – U · be a second-order differential operator on d. Under suitable conditions,L z has the discrete spectrum
- \lambda _2^\varepsilon ...and lim \varepsilon ^2 log \lambda _2^\varepsilon = - \Lambda \hfill \\ \varepsilon \to 0 \hfill \\ \end{gathered} $$ " align="middle" vspace="20%" border="0">  相似文献   
996.
Summary In the present work we extent the results in [RS] on CHIP, i.e. Cardinal Hermite Interpolation by the span of translates of directional derivatives of a box spline. These directional derivatives are that ones which define the type of the Hermite Interpolation. We admit here several (linearly independent) directions with multiplicities instead of one direction as in [RS]. Under the same assumptions on the smoothness of the box spline and its defining matrixT we can prove as in [RS]: CHIP has a system of fundamental solutions which are inL L 2 together with its directional derivatives mentioned above. Moreover, for data sequences inl p ( d ), 1p2, there is a spline function inL p, 1/p+1/p=1, which solves CHIP.Research supported in part by NSERC Canada under Grant # A7687. This research was completed while this author was supported by a grant from the Deutscher Akademischer Austauschdienst  相似文献   
997.
In this paper, assuming a certain set-theoretic hypothesis, a positive answer is given to a question of H. Kraljevi, namely it is shown that there exists a Lebesgue measurable subsetA of the real line such that the set {c R: A + cA contains an interval} is nonmeasurable. Here the setA + cA = {a + ca: a, a A}. Two other results about sets of the formA + cA are presented.  相似文献   
998.
IfK is a field of characteristic 0 then the following is shown. Iff, g, h: M n (K) K are non-constant solutions of the Binet—Pexider functional equation
  相似文献   
999.
The purpose of this paper is to solve the following Pythagorean functional equation:(e p(x,y) ) 2 ) = q(x,y) 2 + r(x, y) 2, where each ofp(x,y), q(x, y) andr(x, y) is a real-valued unknown harmonic function of the real variablesx, y on the wholexy-planeR 2.The result is as follows.  相似文献   
1000.
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