全文获取类型
收费全文 | 4897篇 |
免费 | 334篇 |
国内免费 | 250篇 |
专业分类
化学 | 143篇 |
晶体学 | 2篇 |
力学 | 267篇 |
综合类 | 38篇 |
数学 | 4379篇 |
物理学 | 652篇 |
出版年
2024年 | 24篇 |
2023年 | 114篇 |
2022年 | 220篇 |
2021年 | 298篇 |
2020年 | 347篇 |
2019年 | 214篇 |
2018年 | 260篇 |
2017年 | 309篇 |
2016年 | 249篇 |
2015年 | 270篇 |
2014年 | 425篇 |
2013年 | 378篇 |
2012年 | 309篇 |
2011年 | 263篇 |
2010年 | 221篇 |
2009年 | 230篇 |
2008年 | 170篇 |
2007年 | 165篇 |
2006年 | 141篇 |
2005年 | 122篇 |
2004年 | 64篇 |
2003年 | 45篇 |
2002年 | 67篇 |
2001年 | 117篇 |
2000年 | 44篇 |
1999年 | 88篇 |
1998年 | 67篇 |
1997年 | 43篇 |
1996年 | 38篇 |
1995年 | 20篇 |
1994年 | 9篇 |
1993年 | 15篇 |
1992年 | 11篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 57篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 8篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1936年 | 6篇 |
排序方式: 共有5481条查询结果,搜索用时 31 毫秒
1.
2.
3.
4.
The satisfiability (SAT) problem is a core problem in computer science. Existing studies have shown that most industrial SAT instances can be effectively solved by modern SAT solvers while random SAT instances cannot. It is believed that the structural characteristics of different SAT formula classes are the reasons behind this difference. In this paper, we study the structural properties of propositional formulas in conjunctive normal form (CNF) by the principle of structural entropy of formulas. First, we used structural entropy to measure the complex structure of a formula and found that the difficulty solving the formula is related to the structural entropy of the formula. The smaller the compressing information of a formula, the more difficult it is to solve the formula. Secondly, we proposed a -approximation strategy to approximate the structural entropy of large formulas. The experimental results showed that the proposed strategy can effectively approximate the structural entropy of the original formula and that the approximation ratio is more than 92%. Finally, we analyzed the structural properties of a formula in the solution process and found that a local search solver tends to select variables in different communities to perform the next round of searches during a search and that the structural entropy of a variable affects the probability of the variable being flipped. By using these conclusions, we also proposed an initial candidate solution generation strategy for a local search for SAT, and the experimental results showed that this strategy effectively improves the performance of the solvers CCAsat and Sparrow2011 when incorporated into these two solvers. 相似文献
5.
6.
To improve the numerical evaluation of weakly singular integrals appearing in the boundary element method, a logarithmic Gaussian quadrature formula is usually suggested in the literature. In this formula the singular function is expressed in terms of the distance between source point and field point, which is a real variable. When an anisotropic elastic solid is considered, most of the existing fundamental solutions are written in terms of complex variables. When the problems with holes, cracks, inclusions, or interfaces are considered, to suit for the shape of the boundaries usually a mapping function is introduced and then the solutions are expressed in terms of mapped complex variables. To deal with the trouble induced by the complex variables, in this study through proper change of variables we develop a simple way to improve the evaluation of weakly singular integrals, especially for the problems of anisotropic elastic solids containing holes, cracks, inclusions, or interfaces. By simple matrix expansion, the proposed method is extended to the problems with piezoelectric or magneto-electro-elastic solids. By using the dual reciprocity method, the proposed method employed for the elastostatic fundamental solution can also be applied to the elastodynamic analysis. 相似文献
7.
8.
《European Journal of Operational Research》2006,175(3):1798-1817
Workflow systems provide means and techniques for modelling, designing, performing and controlling repetitive (business) processes. The quality of commercial workflow systems is usually determined to a large extent by their versatility and multi-purpose application. One of the current trends in improving workflow systems lies in enriching modelling methods and techniques in order to enlarge design alternatives.The need for such advanced methods is particularly apparent in those fields in which the process duration can be determined only vaguely, but whose completion schedules are at the same time strictly enforced by a highly competitive market by means of fines and penalties. The risk of an overrun has to be weighed against the expected costs and benefits of certain measures reducing turn-around time and their combinations. Because they can help to avoid such penalties—or, at least, keep any potential losses low by identifying critical subprocesses and evaluate appropriate measures—modelling and evaluation techniques are becoming essential features of workflow systems.Methodologically, we use Stochastic Branch-and-Bound as a technique for finding “optimal” bundles of measures. A numerical study shows the benefits of this meta-approach by means of five stepwise-developed decision scenarios requiring rich modelling. Petri nets as a modelling tool and Stochastic Branch-and-Bound as an optimization technique determine for multi-mode resource constrained workflows of varying complexity an optimal workforce strategy with respect to the number of workers and their qualification. 相似文献
9.
《Journal of Computational and Applied Mathematics》2005,184(1):10-49
We present an overview of some concepts and methodologies we believe useful in modeling HIV pathogenesis. After a brief discussion of motivation for and previous efforts in the development of mathematical models for progression of HIV infection and treatment, we discuss mathematical and statistical ideas relevant to Structured Treatment Interruptions (STI). Among these are model development and validation procedures including parameter estimation, data reduction and representation, and optimal control relative to STI. Results from initial attempts in each of these areas by an interdisciplinary team of applied mathematicians, statisticians and clinicians are presented. 相似文献
10.