全文获取类型
收费全文 | 1725篇 |
免费 | 45篇 |
国内免费 | 13篇 |
专业分类
化学 | 1110篇 |
晶体学 | 5篇 |
力学 | 54篇 |
数学 | 272篇 |
物理学 | 342篇 |
出版年
2023年 | 8篇 |
2022年 | 8篇 |
2021年 | 18篇 |
2020年 | 19篇 |
2019年 | 26篇 |
2018年 | 18篇 |
2017年 | 22篇 |
2016年 | 38篇 |
2015年 | 33篇 |
2014年 | 19篇 |
2013年 | 77篇 |
2012年 | 91篇 |
2011年 | 110篇 |
2010年 | 72篇 |
2009年 | 66篇 |
2008年 | 86篇 |
2007年 | 99篇 |
2006年 | 122篇 |
2005年 | 103篇 |
2004年 | 105篇 |
2003年 | 85篇 |
2002年 | 72篇 |
2001年 | 23篇 |
2000年 | 29篇 |
1999年 | 17篇 |
1998年 | 20篇 |
1997年 | 19篇 |
1996年 | 34篇 |
1995年 | 14篇 |
1994年 | 15篇 |
1993年 | 31篇 |
1992年 | 31篇 |
1991年 | 17篇 |
1990年 | 13篇 |
1989年 | 10篇 |
1988年 | 10篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 15篇 |
1983年 | 11篇 |
1982年 | 16篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 12篇 |
1977年 | 14篇 |
1975年 | 10篇 |
1974年 | 12篇 |
1973年 | 9篇 |
排序方式: 共有1783条查询结果,搜索用时 18 毫秒
1.
2.
Partially supported by NSF Grants. 相似文献
3.
4.
Dmitry I. Belov Ronald D. Armstrong 《Computational Optimization and Applications》2006,33(2-3):319-332
This paper introduces a novel approach for extracting the maximum number of non-overlapping test forms from a large collection
of overlapping test sections assembled from a given item bank. The approach involves solving maximum set packing problems
(MSPs). A branch-and-bound MSP algorithm is developed along with techniques adapted from constraint programming to estimate
lower and upper bounds on the optimal MSP solution. The algorithm is general and can be applied in other applications including
combinatorial auctions. The results of computer simulations and experiments with an operational item bank are presented.
An erratum to this article is available at . 相似文献
5.
For a conformal manifold we introduce the notion of an ambient connection, an affine connection on an ambient manifold of
the conformal manifold, possibly with torsion, and with conditions relating it to the conformal structure. The purpose of
this construction is to realise the normal conformal Tractor holonomy as affine holonomy of such a connection. We give an
example of an ambient connection for which this is the case, and which is torsion free if we start the construction with a
C-space, and in addition Ricci-flat if we start with an Einstein manifold. Thus, for a C-space this example leads to an ambient metric in the weaker sense of Čap and Gover, and for an Einstein space to a Ricci-flat
ambient metric in the sense of Fefferman and Graham.
Current address for first author: Erwin Schr?dinger International Institute for Mathematical Physics (ESI), Boltzmanngasse
9, 1090 Vienna, Austria
Current address for second author: Department of Mathematics, University of Hamburg, Bundesstra?e 55, 20146 Hamburg, Germany 相似文献
6.
7.
In this paper we study the problem of finding a conformal metric with the property that the kth elementary symmetric polynomial of the eigenvalues of its Weyl-Schouten tensor is constant. A new conformal invariant involving maximal volumes is defined, and this invariant is then used in several cases to prove existence of a solution, and compactness of the space of solutions (provided the conformal class admits an admissible metric). In particular, the problem is completely solved in dimension four, and in dimension three if the manifold is not simply connected. 相似文献
8.
Periodic, self-consistent, density functional theory (GGA-PW91) calculations are performed for both surface and subsurface atomic hydrogen on and in Ni(1 1 1). At a low coverage (θ=0.25 ML), the binding energies (BEs) of a hydrogen atom in surface fcc, subsurface octahedral (first layer), and subsurface octahedral (second layer) sites are −2.89, −2.18, and −2.11 eV, respectively. The activation energy barriers for hydrogen diffusion from the surface to the first subsurface layer and from the first to the second subsurface layer are estimated to be 0.88 and 0.52 eV, respectively. In the entire coverage range studied, hydrogen occupies surface fcc and hcp sites and subsurface octahedral sites. In addition, the magnitude of the BE per hydrogen atom and the magnetization of the nickel slabs both decrease as hydrogen coverage increases. Vibrational frequencies of hydrogen at various surface and subsurface sites are calculated and are in reasonable agreement with experimental data. A phase stability calculation with a 2 × 2 surface unit cell shows that a p(2 × 2)-2H overlayer structure (θ=0.5 ML) and a p(1 × 1)-1H structure (θ=1.0 ML) are stable at low hydrogen pressures, in agreement with numerous experimental results. A very large increase in pressure is required to populate subsurface sites. After such an increase occurs, the first subsurface layer is filled completely. 相似文献
9.
In this paper we consider the problem of estimating an unknown joint distribution which is defined over mixed discrete and continuous variables. A nonparametric kernel approach is proposed with smoothing parameters obtained from the cross-validated minimization of the estimator's integrated squared error. We derive the rate of convergence of the cross-validated smoothing parameters to their ‘benchmark’ optimal values, and we also establish the asymptotic normality of the resulting nonparametric kernel density estimator. Monte Carlo simulations illustrate that the proposed estimator performs substantially better than the conventional nonparametric frequency estimator in a range of settings. The simulations also demonstrate that the proposed approach does not suffer from known limitations of the likelihood cross-validation method which breaks down with commonly used kernels when the continuous variables are drawn from fat-tailed distributions. An empirical application demonstrates that the proposed method can yield superior predictions relative to commonly used parametric models. 相似文献
10.
Jeff D. Kahn Nathan Linial Noam Nisan Michael E. Saks 《Journal of Theoretical Probability》1989,2(1):121-128
This article deals with random walks on arbitrary graphs. We consider the cover time of finite graphs. That is, we study the expected time needed for a random walk on a finite graph to visit every vertex at least once. We establish an upper bound ofO(n
2) for the expectation of the cover time for regular (or nearly regular) graphs. We prove a lower bound of (n logn) for the expected cover time for trees. We present examples showing all our bounds to be tight.Mike Saks was supported by NSF-DMS87-03541 and by AFOSR-0271. Jeff Kahn was supported by MCS-83-01867 and by AFOSR-0271. 相似文献