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   21712篇
  免费   2600篇
  国内免费   594篇
化学   1878篇
晶体学   44篇
力学   1705篇
综合类   194篇
数学   4898篇
物理学   3279篇
无线电   12908篇
  2024年   35篇
  2023年   241篇
  2022年   365篇
  2021年   511篇
  2020年   523篇
  2019年   359篇
  2018年   364篇
  2017年   727篇
  2016年   895篇
  2015年   970篇
  2014年   1608篇
  2013年   1499篇
  2012年   1569篇
  2011年   1508篇
  2010年   1165篇
  2009年   1248篇
  2008年   1397篇
  2007年   1350篇
  2006年   1220篇
  2005年   1030篇
  2004年   897篇
  2003年   991篇
  2002年   679篇
  2001年   539篇
  2000年   488篇
  1999年   419篇
  1998年   347篇
  1997年   273篇
  1996年   251篇
  1995年   228篇
  1994年   183篇
  1993年   136篇
  1992年   96篇
  1991年   102篇
  1990年   71篇
  1989年   57篇
  1988年   60篇
  1987年   48篇
  1986年   40篇
  1985年   64篇
  1984年   36篇
  1983年   29篇
  1982年   33篇
  1981年   26篇
  1980年   47篇
  1979年   44篇
  1978年   26篇
  1977年   35篇
  1976年   36篇
  1974年   13篇
排序方式: 共有10000条查询结果,搜索用时 765 毫秒
991.
This paper shows how the dynamic program algorithm called the Q algorithm can be used as an alternative algorithm to produce the coefficients of a least squares problem. It shows also how the output of the algorithm can be used to calculate various statistical quantities needed to evaluate linear models. In particular, we show how to calculate standard statistical quantities like the coefficient of determination R2, the t statistics, and the F statistics. These quantities serve as a measure of how well the model fits the data.  相似文献   
992.
The general theory of approximation of (possibly generalized) Young measures is presented, and concrete cases are investigated. An adjoint-operator approach, combined with quasi-interpolation of test integrands, is systematically used. Applicability is demonstrated on an optimal control problem for an elliptic system, together with one-dimensional illustrative calculations of various options.  相似文献   
993.
   Abstract. Optimal control problems governed by semilinear parabolic partial differential equations are considered. No Cesari-type conditions are assumed. By proving the existence theorem and the Pontryagin maximum principle of optimal ``state-control" pairs for the corresponding relaxed problems, an existence theorem of optimal pairs for the original problem is established.  相似文献   
994.
A process is considered whose quality deteriorates accordingto a constant failure intensity . As in practice it can be difficultto estimate the true value of the purpose of this paper isto present a strategy which can be applied without knowing .In order to maximize the number of conforming items per timeunit perfect inspections and renewals are performed. The lengthof the inspection interval is described by an arithmetical sequenceand changes by the time depending on perceived assignable causes.Optimal adaptive control plans provide nearly the same performanceas in the case when is known.  相似文献   
995.
1. PreliminaryIt is well known that{1] a 8ymPlectic form is invariant along the trajectory of a Hamilto-nian system. Based on this fundamental property, certain techniques have been developed.The purpose of this paper is to extend such an approach to a wider class of dynamic systeIns,namely, genera1ized Hamiltonian systems. Our purpose is to investigate a class of dynaInicsystems, which possess a certain "geometric structure".Deflnition 1.1[1'2]. Let M be a tIlallifo1d. w E fl'(M) is call…  相似文献   
996.
Let f: XY be a nonlinear differentiable map, X,Y are Hilbert spaces, B(a,r) is a ball in X with a center a and radius r. Suppose f (x) is Lipschitz in B(a,r) with Lipschitz constant L and f (a) is a surjection: f (a)X=Y; this implies the existence of >0 such that f (a)* yy, yY. Then, if r,/(2L), the image F=f(B(a,)) of the ball B(a,) is convex. This result has numerous applications in optimization and control. First, duality theory holds for nonconvex mathematical programming problems with extra constraint xa. Special effective algorithms for such optimization problems can be constructed as well. Second, the reachability set for small power control is convex. This leads to various results in optimal control.  相似文献   
997.
Gautam  N.  Kulkarni  V.G. 《Queueing Systems》2000,36(4):351-379
In this paper, we consider the stochastic fluid-flow model of a single node in a high-speed telecommunication network handling multi-class traffic. The node has multiple buffers, one for each class of traffic. The contents of these buffers are multiplexed onto a single output channel using one of the service scheduling policies: the Timed Round Robin Policy or the Static Priority Service Policy. The Quality of Service requirements for each class are based on cell loss probabilities. Using effective bandwidth methodologies and the recently developed bounds for semi-Markov modulated traffic, we solve call admission control problems for the two service scheduling policies at this node. We compare the performance of the effective bandwidth methodologies and the SMP bounds technique. We also numerically compare the performance of the two service scheduling policies.  相似文献   
998.
This paper presents a unified framework for pull production control mechanisms in multistage manufacturing systems. A pull production control mechanism in a multistage manufacturing system is a mechanism that coordinates the release of parts into each stage of the system with the arrival of customer demands for final products. Four basic pull production control mechanisms are presented: Base Stock, Kanban, Generalized Kanban, and Extended Kanban. It is argued that on top of any of these basic coordination mechanisms, a local mechanism to control the workinprocess in each stage may be superimposed. Several cases of basic stage coordination mechanisms with stage workinprocess control are presented, and several production control systems that have appeared in the literature are shown to be equivalent to some of these cases.  相似文献   
999.
In this paper the time-optimal control problem for a distributedparabolic system in which time lags appear in the integral formboth in the state equation and in the boundary condition ispresented. The particular properties of the optimal controlare discussed.  相似文献   
1000.
Langford系统Hopf分叉的线性反馈控制   总被引:3,自引:0,他引:3       下载免费PDF全文
刘素华  唐驾时 《物理学报》2007,56(6):3145-3151
分析Langford系统的Hopf分叉现象,并研究采用线性反馈控制方法控制该系统的Hopf分叉.从理论上推导出受控系统产生Hopf分叉的条件,给出了某些极限环的解析表达式,对该系统进行了分叉点的转移与极限环的稳定性控制.数值模拟说明本文采用的方法对Langford系统的Hopf分叉控制是有效的. 关键词: Langford系统 反馈控制 Hopf分叉 极限环  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