全文获取类型
收费全文 | 398634篇 |
免费 | 5353篇 |
国内免费 | 1454篇 |
专业分类
化学 | 218401篇 |
晶体学 | 5823篇 |
力学 | 16556篇 |
综合类 | 12篇 |
数学 | 46202篇 |
物理学 | 118447篇 |
出版年
2020年 | 2462篇 |
2019年 | 2405篇 |
2018年 | 2421篇 |
2016年 | 4758篇 |
2015年 | 3955篇 |
2014年 | 5374篇 |
2013年 | 17282篇 |
2012年 | 12929篇 |
2011年 | 16219篇 |
2010年 | 10060篇 |
2009年 | 9907篇 |
2008年 | 14892篇 |
2007年 | 15152篇 |
2006年 | 14835篇 |
2005年 | 13732篇 |
2004年 | 12309篇 |
2003年 | 10930篇 |
2002年 | 10689篇 |
2001年 | 12167篇 |
2000年 | 9376篇 |
1999年 | 7504篇 |
1998年 | 6153篇 |
1997年 | 5975篇 |
1996年 | 5985篇 |
1995年 | 5549篇 |
1994年 | 5164篇 |
1993年 | 4980篇 |
1992年 | 5592篇 |
1991年 | 5439篇 |
1990年 | 5088篇 |
1989年 | 4932篇 |
1988年 | 5191篇 |
1987年 | 4917篇 |
1986年 | 4726篇 |
1985年 | 6800篇 |
1984年 | 6926篇 |
1983年 | 5671篇 |
1982年 | 6305篇 |
1981年 | 6205篇 |
1980年 | 5980篇 |
1979年 | 6053篇 |
1978年 | 6201篇 |
1977年 | 6115篇 |
1976年 | 6084篇 |
1975年 | 5913篇 |
1974年 | 5647篇 |
1973年 | 5918篇 |
1972年 | 3496篇 |
1971年 | 2627篇 |
1968年 | 2577篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Genetic algorithms are stochastic search algorithms that have been applied to optimization problems. In this paper we analyze the run-time complexity of a genetic algorithm when we are interested in one of a set of distinguished solutions. One such case occurs when multiple optima exist. We define the worst case scenario and derive a probabilistic worst case bound on the number of iterations required to find one of these multiple solutions of interest. 相似文献
992.
993.
994.
995.
996.
Q.J. Zhu 《Journal of Mathematical Analysis and Applications》2007,326(1):708-720
Investment systems are studied using a framework that emphasize their profiles (the cumulative probability distribution on all the possible percentage gains of trades) and their log return functions (the expected average return per trade in logarithmic scale as a function of the investment size in terms of the percentage of the available capital). The efficiency index for an investment system, defined as the maximum of the log return function, is proposed as a measure to compare investment systems for their intrinsic merit. This efficiency index can be viewed as a generalization of Shannon's information rate for a communication channel. Applications are illustrated. 相似文献
997.
The title of this paper states precisely what the subject is. The first part of the paper concerns the radially-symmetric
problem in the exterior of the unit ball. It is shown that in time the solution of the problem converges to one of two specific
self-similar solutions of the porous media equation, dependent upon the dimensionality of the problem. Moreover, the free
boundary of the solution converges to that of the self-similar solution. The critical space dimension is two, for which there
is no distinction between the self-similar solutions, and the form of the convergence is exceptional. The technique used is
a comparison principle involving a variable that is a weighted integral of the solution. The second part of the paper is devoted
to the problem in an arbitrary spatial domain with no conditions of symmetry. A special invariance principle and the results
obtained for the radially-symmetric case are used to determine the large-time behaviour of solutions and their free boundaries.
This behaviour is decidedly different from when the boundary data are fixed and not homogeneous. 相似文献
998.
W.G. Schmidt 《Applied Physics A: Materials Science & Processing》2002,75(1):89-99
There has been renewed interest in the structure of III-V compound semiconductor (001) surfaces caused by recent experimental
and theoretical findings, which indicate that geometries different from the seemingly well-established dimer models describe
the surface ground state for specific preparation conditions. I review briefly the structure information available on the
(001) surfaces of GaP, InP, GaAs and InAs. These data are complemented with first-principles total-energy calculations. The
calculated surface phase diagrams are used to explain the experimental data and reveal that the stability of specific surface
structures depends largely on the relative size of the surface constituents. Several structural models for the Ga-rich GaAs (001)(4×6)
surface are discussed, but dismissed on energetic grounds. I discuss in some detail the electronic properties of the recently
proposed cation-rich GaAs (001)ζ(4×2) geometry.
Received: 18 May 2001 / Revised version: 23 July 2001 / Published online: 3 April 2002 相似文献
999.
We demonstrate the surprising integrability of the classical Hamiltonian associated to a spin 1/2 system under periodic external fields. The one-qubit rotations generated by the dynamical evolution is, on the one hand, close to that of the rotating wave approximation (RWA), on the other hand to two different “average” systems, according to whether a certain parameter is small or large. Of particular independent interest is the fact that both the RWA and the averaging theorem are seen to hold well beyond their expected region of validity. Finally, we determine conditions for the realization of the quantum NOT operation by means of classical stroboscopic maps. 相似文献
1000.
C. Bréchignac Ph. Cahuzac F. Carlier C. Colliex M. de Frutos N. Kébaïli J. Le Roux A. Masson B. Yoon 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,24(1-3):265-268
We studied shape relaxation of nano-fractal islands,
during annealing, after their growth from antimony cluster
deposition on graphite surface. Annealing at
180°C shows evidence of an increase
of the fractal branch width with time followed by branch
fragmentation, without changing the fractal dimension. The time
evolution of the width of the arm suggests the surface
self-diffusion mechanism as the main relaxation process. With
Monte Carlo simulations, we confirmed the observed behavior.
Comparison is done with our previous results on fragmentation of
nano-fractal silver islands when impurity added to the incident
cluster promotes rapid fragmentation by surface self-diffusion
enhancement [1]. 相似文献