全文获取类型
收费全文 | 13588篇 |
免费 | 1680篇 |
国内免费 | 394篇 |
专业分类
化学 | 1454篇 |
晶体学 | 30篇 |
力学 | 1642篇 |
综合类 | 191篇 |
数学 | 8972篇 |
物理学 | 3373篇 |
出版年
2024年 | 17篇 |
2023年 | 126篇 |
2022年 | 247篇 |
2021年 | 413篇 |
2020年 | 263篇 |
2019年 | 261篇 |
2018年 | 284篇 |
2017年 | 390篇 |
2016年 | 396篇 |
2015年 | 297篇 |
2014年 | 602篇 |
2013年 | 770篇 |
2012年 | 817篇 |
2011年 | 683篇 |
2010年 | 685篇 |
2009年 | 832篇 |
2008年 | 892篇 |
2007年 | 952篇 |
2006年 | 804篇 |
2005年 | 680篇 |
2004年 | 512篇 |
2003年 | 528篇 |
2002年 | 479篇 |
2001年 | 454篇 |
2000年 | 403篇 |
1999年 | 322篇 |
1998年 | 345篇 |
1997年 | 309篇 |
1996年 | 221篇 |
1995年 | 232篇 |
1994年 | 196篇 |
1993年 | 172篇 |
1992年 | 156篇 |
1991年 | 127篇 |
1990年 | 106篇 |
1989年 | 85篇 |
1988年 | 74篇 |
1987年 | 63篇 |
1986年 | 62篇 |
1985年 | 78篇 |
1984年 | 64篇 |
1983年 | 22篇 |
1982年 | 36篇 |
1981年 | 32篇 |
1980年 | 26篇 |
1979年 | 37篇 |
1978年 | 28篇 |
1977年 | 34篇 |
1976年 | 22篇 |
1957年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Dual fractional cutting plane algorithms, in which cutting planes are used to iteratively tighten a linear relaxation of an integer program,
are well-known and form the basis of the highly successful branch-and-cut method. It is rather less well-known that various primal cutting plane algorithms were developed in the 1960s, for example by Young. In a primal algorithm, the main role of the cutting
planes is to enable a feasible solution to the original problem to be improved. Research on these algorithms has been almost
non-existent.
In this paper we argue for a re-examination of these primal methods. We describe a new primal algorithm for pure 0-1 problems based on strong valid inequalities and give some encouraging computational results. Possible extensions to the case of general
mixed-integer programs are also discussed. 相似文献
22.
23.
This paper is concerned with the implementation and testing of an algorithm for solving constrained least-squares problems. The algorithm is an adaptation to the least-squares case of sequential quadratic programming (SQP) trust-region methods for solving general constrained optimization problems. At each iteration, our local quadratic subproblem includes the use of the Gauss–Newton approximation but also encompasses a structured secant approximation along with tests of when to use this approximation. This method has been tested on a selection of standard problems. The results indicate that, for least-squares problems, the approach taken here is a viable alternative to standard general optimization methods such as the Byrd–Omojokun trust-region method and the Powell damped BFGS line search method. 相似文献
24.
具有模糊数的模糊多目标群体决策优选模型与方法 总被引:5,自引:0,他引:5
多目标群体决策问题是运筹学的一个重要研究领域,目前已经提出了一些有效的决策方法。但对目标值和权重均为模糊数的模糊多目标群体决策问题却研究不多,本对此类模糊多目标群体决策问题进行了探讨,利用相对正理想方案与相对负理想方案概念定义了相对差异距离,进而建立了模糊多目标群体决策优选模型与方法,并通过战役决心方案的评价说明了该方法是可行、有效的,可作为军事决策与决策支持系统的备选方法。 相似文献
25.
Evolutionary algorithms are applied as problem-independent optimization algorithms. They are quite efficient in many situations. However, it is difficult to analyze even the behavior of simple variants of evolutionary algorithms like the (1+1) EA on rather simple functions. Nevertheless, only the analysis of the expected run time and the success probability within a given number of steps can guide the choice of the free parameters of the algorithms. Here static (1+1) EAs with a fixed mutation probability are compared with dynamic (1+1) EAs with a simple schedule for the variation of the mutation probability. The dynamic variant is first analyzed for functions typically chosen as example-functions for evolutionary algorithms. Afterwards, it is shown that it can be essential to choose the suitable variant of the (1+1) EA. More precisely, functions are presented where each static (1+1) EA has exponential expected run time while the dynamic variant has polynomial expected run time. For other functions it is shown that the dynamic (1+1) EA has exponential expected run time while a static (1+1) EA with a good choice of the mutation probability has polynomial run time with overwhelming probability. 相似文献
26.
27.
As borders between different entities, lines are an important element of natural images. Indeed, the neurons of the mammalian
visual cortex are tuned to respond best to lines of a given orientation. This preferred orientation varies continuously across
most of the cortex, but also has vortex-like singularities known as pinwheels. In attempting to describe such patterns of
orientation preference, we are led to consider underlying rotation symmetries: Oriented segments in natural images tend to
be collinear; neurons are more likely to be connected if their preferred orientations are aligned to their topographic separation.
These are indications of a reduced symmetry requiring joint rotations of both orientation preference and the underlying topography.
This is verified by direct statistical tests in both natural images and in cortical maps. Using the statistics of natural
scenes we construct filters that are best suited to extracting information from such images, and find qualitative similarities
to mammalian vision.
PACS84.35+i 89.70.+c 87.57.Nk 相似文献
28.
29.
It is long known that the Fokker-Planck equation with prescribed constant coefficients of diffusion and linear friction describes
the ensemble average of the stochastic evolutions in velocity space of a Brownian test particle immersed in a heat bath of
fixed temperature. Apparently, it is not so well known that the same partial differential equation, but now with constant
coefficients which are functionals of the solution itself rather than being prescribed, describes the kinetic evolution (in
the N→∞ limit) of an
isolated
N-particle system with certain stochastic interactions. Here we discuss in detail this recently discovered interpretation.
An erratum to this article can be found at 相似文献
30.
This paper gives some global and uniform convergence estimates for a class of subspace correction (based on space decomposition) iterative methods applied to some unconstrained convex optimization problems. Some multigrid and domain decomposition methods are also discussed as special examples for solving some nonlinear elliptic boundary value problems.