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   458660篇
  免费   6708篇
  国内免费   2673篇
化学   250964篇
晶体学   6641篇
力学   19428篇
综合类   27篇
数学   53672篇
物理学   137309篇
  2021年   4353篇
  2020年   4657篇
  2019年   5000篇
  2018年   6353篇
  2017年   6200篇
  2016年   9734篇
  2015年   6247篇
  2014年   9323篇
  2013年   21941篇
  2012年   17423篇
  2011年   21208篇
  2010年   14631篇
  2009年   14632篇
  2008年   19074篇
  2007年   18917篇
  2006年   17486篇
  2005年   15766篇
  2004年   14922篇
  2003年   12963篇
  2002年   12665篇
  2001年   13813篇
  2000年   10375篇
  1999年   8152篇
  1998年   6805篇
  1997年   6512篇
  1996年   6303篇
  1995年   5633篇
  1994年   5541篇
  1993年   5387篇
  1992年   5992篇
  1991年   6019篇
  1990年   5745篇
  1989年   5550篇
  1988年   5563篇
  1987年   5454篇
  1986年   5146篇
  1985年   6693篇
  1984年   6800篇
  1983年   5372篇
  1982年   5478篇
  1981年   5455篇
  1980年   5041篇
  1979年   5423篇
  1978年   5549篇
  1977年   5438篇
  1976年   5387篇
  1975年   4962篇
  1974年   4933篇
  1973年   4919篇
  1972年   3472篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
[Theg-factor ratio of the first excited 3? and 5? states in40Ca was measured to beg 3/g 5=1.01(10) employing the implantation perturbed angular correlation technique. The static hyperfine fields (SF) in Fe and Gd hosts were used. In addition the lifetime of the 5? state was measured to be τ=426(7)ps. The values of the SF in Gd and Fe hosts were deduced and compared with systematics in this element region.  相似文献   
992.
993.
994.
The issues involved in teaching English language learners mathematics while they are learning English pose many challenges for mathematics teachers and highlight the need to focus on language-processing issues related to teaching mathematical content. Two realistic-type problems from high-stakes tests are used to illustrate the complex interactions between culture, language, and mathematical learning. The analyses focus on aspects of the problems that potentially increase cognitive demands for second-language learners. An analytical framework is presented that is designed to enable mathematics teachers to identify critical elements in problems and the learning environment that contribute to increased cognitive demands for students of English as a second language. The framework is proposed as a cycle of teacher reflection that would extend a constructivist model of teaching to include broader linguistic, cultural, and cognitive processing issues of mathematics teaching, as well as enable teachers to develop more accurate mental models of student learning.  相似文献   
995.
We establish constructive existence conditions and construct a generalized Green operator for the construction of solutions of a Noetherian linear boundary-value problem for a system of ordinary differential equations with switchings and pulse action in critical and noncritical cases. __________ Translated from Neliniini Kolyvannya, Vol. 10, No. 1, pp. 51–65, January–March, 2007.  相似文献   
996.
The surgery obstruction of a normal map to a simple Poincaré pair (X, Y) lies in the relative surgery obstruction group L *(π 1(Y) → π 1(X)). A well-known result of Wall, the so-called π-π-theorem, states that in higher dimensions a normal map of a manifold with boundary to a simple Poincaré pair with π 1(X) ? π 1(Y) is normally bordant to a simple homotopy equivalence of pairs. In order to study normal maps to a manifold with a submanifold, Wall introduced the surgery obstruction groups LP * for manifold pairs and splitting obstruction groups LS *. In the present paper, we formulate and prove for manifold pairs with boundary results similar to the π-π-theorem. We give direct geometric proofs, which are based on the original statements of Wall’s results and apply obtained results to investigate surgery on filtered manifolds.  相似文献   
997.
We study linear and nonlinear simultaneous Fourier-Pade approximation for Angelesco systems of functions and give the exact rate of convergence/divergence of the approximants in terms of the solution of associated vector equilibrium potential problems which differ for the linear and nonlinear cases.  相似文献   
998.
We consider queuing systems where customers are not allowed to queue, instead of that they make repeated attempts, or retrials, in order to enter service after some time. We obtain the distribution of the number of retrials produced by a tagged customer, until he finds an available server.  相似文献   
999.
Many recent algorithmic approaches involve the construction of a differential equation model for computational purposes, typically by introducing an artificial time variable. The actual computational model involves a discretization of the now time-dependent differential system, usually employing forward Euler. The resulting dynamics of such an algorithm is then a discrete dynamics, and it is expected to be “close enough” to the dynamics of the continuous system (which is typically easier to analyze) provided that small – hence many – time steps, or iterations, are taken. Indeed, recent papers in inverse problems and image processing routinely report results requiring thousands of iterations to converge. This makes one wonder if and how the computational modeling process can be improved to better reflect the actual properties sought. In this article we elaborate on several problem instances that illustrate the above observations. Algorithms may often lend themselves to a dual interpretation, in terms of a simply discretized differential equation with artificial time and in terms of a simple optimization algorithm; such a dual interpretation can be advantageous. We show how a broader computational modeling approach may possibly lead to algorithms with improved efficiency. AMS subject classification (2000)  65L05, 65M32, 65N21, 65N22, 65D18  相似文献   
1000.
In cricket, a rain-affected pitch can make batting more difficult than normal. Several other conditions such as poor light or an initially lively pitch, may also result in difficulties for the batsmen. In this note, we refer to all of them as ‘sticky wickets’. On sticky wickets, lower order batsmen are often sent into ‘hold the fort’ until conditions improve. In this paper, a stochastic dynamic programming model is used to examine the appropriateness of this policy. The model suggests that the tactic is often optimal when the sticky wicket persists until the end of the day's play, but not often when the sticky wicket is transitory. In some circumstances, it is worthwhile, on a normal wicket near the end of the day, to send in a lower order batsman to hold the fort (a night watchman): when the wicket is sticky, this tactic is even more worthwhile.  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