全文获取类型
收费全文 | 12028篇 |
免费 | 1834篇 |
国内免费 | 577篇 |
专业分类
化学 | 1913篇 |
晶体学 | 46篇 |
力学 | 1871篇 |
综合类 | 231篇 |
数学 | 6853篇 |
物理学 | 3525篇 |
出版年
2023年 | 161篇 |
2022年 | 199篇 |
2021年 | 329篇 |
2020年 | 302篇 |
2019年 | 275篇 |
2018年 | 285篇 |
2017年 | 500篇 |
2016年 | 581篇 |
2015年 | 501篇 |
2014年 | 620篇 |
2013年 | 913篇 |
2012年 | 643篇 |
2011年 | 652篇 |
2010年 | 575篇 |
2009年 | 692篇 |
2008年 | 764篇 |
2007年 | 741篇 |
2006年 | 618篇 |
2005年 | 494篇 |
2004年 | 472篇 |
2003年 | 500篇 |
2002年 | 435篇 |
2001年 | 340篇 |
2000年 | 347篇 |
1999年 | 327篇 |
1998年 | 276篇 |
1997年 | 215篇 |
1996年 | 194篇 |
1995年 | 188篇 |
1994年 | 141篇 |
1993年 | 104篇 |
1992年 | 92篇 |
1991年 | 101篇 |
1990年 | 85篇 |
1989年 | 61篇 |
1988年 | 75篇 |
1987年 | 61篇 |
1986年 | 46篇 |
1985年 | 77篇 |
1984年 | 49篇 |
1983年 | 35篇 |
1982年 | 48篇 |
1981年 | 33篇 |
1980年 | 57篇 |
1979年 | 53篇 |
1978年 | 32篇 |
1977年 | 45篇 |
1976年 | 47篇 |
1974年 | 15篇 |
1957年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
61.
反对称正交对称矩阵反问题 总被引:6,自引:0,他引:6
本文讨论一类反对称正交对称矩阵反问题及其最佳逼近.研究了这类矩阵的一些性质,利用这些性质给出了反问题解存在的一些条件和解的一般表达式,不仅证明了最佳逼近解的存在唯一性,而且给出了此解的具体表达式. 相似文献
62.
63.
基于改进遗传算法的布局优化子问题 总被引:2,自引:0,他引:2
本针对子问题,构造了布局子问题(关于同构布局等价类)的改进遗传算法。将该算法应用于二维布局优化子问题,数值实验表明该算法能够在很好地保持图元的邻接关系的前提下找到子问题的最优解。由于布局优化问题可分解为有限个子问题,所以利用该算法可以找到整个布局优化问题的全局最优解。 相似文献
64.
65.
本文首先提出逆(反)对策这一新问题,给出了数学模型;探讨了“奇门遁甲”预测理论(术)中的数学问题;通过系统分析“专门遁甲”预测过程,可知它的预测过程隐含着一个特殊的逆(反)对策问题;最后指出逆(反)对策问题的广泛存在并给出案例分析. 相似文献
66.
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
67.
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
68.
Efficient Sequential Quadratic Programming Implementations for Equality-Constrained Discrete-Time Optimal Control 总被引:1,自引:0,他引:1
Efficient sequential quadratic programming (SQP) implementations are presented for equality-constrained, discrete-time, optimal control problems. The algorithm developed calculates the search direction for the equality-based variant of SQP and is applicable to problems with either fixed or free final time. Problem solutions are obtained by solving iteratively a series of constrained quadratic programs. The number of mathematical operations required for each iteration is proportional to the number of discrete times N. This is contrasted by conventional methods in which this number is proportional to N
3. The algorithm results in quadratic convergence of the iterates under the same conditions as those for SQP and simplifies to an existing dynamic programming approach when there are no constraints and the final time is fixed. A simple test problem and two application problems are presented. The application examples include a satellite dynamics problem and a set of brachistochrone problems involving viscous friction. 相似文献
69.
UCINSKI DARIUSZ; JAI ABDELHAQ EL 《IMA Journal of Mathematical Control and Information》1997,14(2):153-174
In this paper, the concept of weak spreadability is introduced.It constitutes an extension of the idea developed by El Jai& Kassara. In the case of linear distributed systems weconsider quadratic control techniques with a conveniently penalizedcriterion which makes the system weakly spreadable. The approachis outlined for a convectiondiffusion system, and theresults of a numerical study are also included to illustratethe main features of the considered problem. 相似文献
70.
A new scheme for controlling photodissociation through preparation of a variationally optimized linear superposition of field free vibrational eigenstates is applied for selective control of IBr and HI dissociation. The dependence of photodissociation on various field parameters and initial conditions is examined to investigate the mechanistic basis of selective control. The parametric equations of motion approach for determining vibrational dynamics as a function of field parameters without having to solve the time dependent Schrödinger equation explicitly for each field parameter separately is outlined and its use to identify field characteristics which will provide the requisite population mix represented by the optimal linear superposition of vibrational states is advocated. 相似文献