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   22530篇
  免费   2829篇
  国内免费   1504篇
化学   5538篇
晶体学   72篇
力学   2310篇
综合类   339篇
数学   13742篇
物理学   4862篇
  2024年   26篇
  2023年   271篇
  2022年   297篇
  2021年   455篇
  2020年   578篇
  2019年   534篇
  2018年   558篇
  2017年   761篇
  2016年   1002篇
  2015年   839篇
  2014年   1143篇
  2013年   1708篇
  2012年   1191篇
  2011年   1237篇
  2010年   1099篇
  2009年   1412篇
  2008年   1553篇
  2007年   1433篇
  2006年   1234篇
  2005年   1047篇
  2004年   917篇
  2003年   1003篇
  2002年   861篇
  2001年   623篇
  2000年   651篇
  1999年   630篇
  1998年   500篇
  1997年   507篇
  1996年   335篇
  1995年   305篇
  1994年   255篇
  1993年   179篇
  1992年   155篇
  1991年   156篇
  1990年   148篇
  1989年   117篇
  1988年   116篇
  1987年   100篇
  1986年   82篇
  1985年   121篇
  1984年   95篇
  1983年   51篇
  1982年   69篇
  1981年   61篇
  1980年   85篇
  1979年   70篇
  1978年   57篇
  1977年   74篇
  1976年   60篇
  1974年   27篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
工程施工中运输网络优化方案   总被引:5,自引:0,他引:5  
施工工地运输方案的优化设计可以归结为 :按施工期要求设计运输线路或验证已有线路通行能力 ,计算总线路中影响提高流量的关键路段 ,取得最小费用最大流 .本文运用图论理论这一数学工具把实际问题抽象为有向网络 ,进而建立数学模型 .此方法理论上严密 ,解题步聚直观清晰 ,对水利、公路、水路、铁路等其它运输系统有普遍意义 .  相似文献   
92.
Inequalities satisfied by the zeros of the solutions of second-order hypergeometric equations are derived through a systematic use of Liouville transformations together with the application of classical Sturm theorems. This systematic study allows us to improve previously known inequalities and to extend their range of validity as well as to discover inequalities which appear to be new. Among other properties obtained, Szegő's bounds on the zeros of Jacobi polynomials for , are completed with results for the rest of parameter values, Grosjean's inequality (J. Approx. Theory 50 (1987) 84) on the zeros of Legendre polynomials is shown to be valid for Jacobi polynomials with |β|1, bounds on ratios of consecutive zeros of Gauss and confluent hypergeometric functions are derived as well as an inequality involving the geometric mean of zeros of Bessel functions.  相似文献   
93.
94.
This paper deals with maximum principle for some optimal control problem governed by some elliptic variational inequalities. Some state constraints are discussed. The basic techniques used here are based on those in [1] and a new penalty functional defined in this paper.  相似文献   
95.
空间有源消声的微机控制   总被引:1,自引:0,他引:1       下载免费PDF全文
本文讨论用通用微机控制空间有源消声.以修正的PID(比例、积分、微分)算法加上逻辑判断构成的控制软件,使得系统收敛迅速,跟踪速度快,消声效果令人满意,而且系统工作稳定可靠,  相似文献   
96.
本文得到了广义区间空间中几个参数型非空交定理,并用此结果证明了一些新型极大极小定理,截口定理,重合定理,以及变分不等式解的存在性定理。本文中的结果包含(1,3,5-12,14-16)中相应结果。  相似文献   
97.
Using a method of uniform approximations, necessary and sufficient conditions for a nonsmooth constrained vector-valued minimax problem are established in terms of Mordukhovich subdifferentials.  相似文献   
98.
The first aim in the present paper is to give an integral representation for Beppo Levi functions on R n. Our integral representation is an extension of Sobolev's integral representation given for infinitely differentiable functions with compact support. As applications, continuity and differentiability properties of Beppo Levi functions are studied.Our second aim in this paper is to study the existence of limits at infinity for Beppo Levi functions. We also consider the existence of fine-type limits at infinity with respect to Bessel capacities, which yields the radial limit result at infinity.  相似文献   
99.
Fully and partially observed stochastic control of systems with nonlinear dynamics and terminal and running costs are considered. Measure changes are introduced which allow both state and observation dynamics to be thought of as linear. In the case when the terms of the cost have a special form the measure change transformation “cancels out” the nonlinearities and changes the original nonlinear problem into a classical LQG one and standard results can be applied. We also consider unnormalized conditional densities of the whole path as state variables and obtain dynamic programming and verification results. R. J. Elliott wishes to acknowledge support of the Natural Sciences and Engineering Research Council of Canada, Grant A7964.  相似文献   
100.
We develop a production policy that controls work-in-process (WIP) levels and satisfies demand in a multistage manufacturing system with significant uncertainty in yield, rework, and demand. The problem addressed in this paper is more general than those in the literature in three aspects: (i) multiple products are processed at multiple workstations, and the capacity of each workstation is limited and shared by multiple operations; (ii) the behavior of a production policy is investigated over an infinite-time horizon, and thus the system stability can be evaluated; (iii) the representation of yield and rework uncertainty is generalized. Generalizing both the system structure and the nature of uncertainty requires a new mathematical development in the theory of infinite-horizon stochastic dynamic programming. The theoretical contributions of this paper are the existence proofs of the optimal stationary control for a stochastic dynamic programming problem and the finite covariances of WIP and production levels under the general expression of uncertainty. We develop a simple and explicit sufficient condition that guarantees the existence of both the optimal stationary control and the system stability. We describe how a production policy can be constructed for the manufacturing system based on the propositions derived.  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