全文获取类型
收费全文 | 220411篇 |
免费 | 2083篇 |
国内免费 | 480篇 |
专业分类
化学 | 117993篇 |
晶体学 | 3191篇 |
力学 | 9654篇 |
综合类 | 7篇 |
数学 | 22767篇 |
物理学 | 69362篇 |
出版年
2021年 | 1770篇 |
2020年 | 1960篇 |
2019年 | 2189篇 |
2018年 | 2924篇 |
2017年 | 2989篇 |
2016年 | 4396篇 |
2015年 | 2462篇 |
2014年 | 3956篇 |
2013年 | 9246篇 |
2012年 | 7471篇 |
2011年 | 9088篇 |
2010年 | 6587篇 |
2009年 | 6617篇 |
2008年 | 8317篇 |
2007年 | 8225篇 |
2006年 | 7799篇 |
2005年 | 7064篇 |
2004年 | 6345篇 |
2003年 | 5667篇 |
2002年 | 5733篇 |
2001年 | 6248篇 |
2000年 | 4715篇 |
1999年 | 3641篇 |
1998年 | 3226篇 |
1997年 | 3105篇 |
1996年 | 2785篇 |
1995年 | 2632篇 |
1994年 | 2612篇 |
1993年 | 2457篇 |
1992年 | 2762篇 |
1991年 | 2938篇 |
1990年 | 2705篇 |
1989年 | 2733篇 |
1988年 | 2590篇 |
1987年 | 2440篇 |
1986年 | 2340篇 |
1985年 | 2982篇 |
1984年 | 3166篇 |
1983年 | 2656篇 |
1982年 | 2830篇 |
1981年 | 2621篇 |
1980年 | 2528篇 |
1979年 | 2705篇 |
1978年 | 2994篇 |
1977年 | 2874篇 |
1976年 | 2831篇 |
1975年 | 2711篇 |
1974年 | 2773篇 |
1973年 | 2734篇 |
1972年 | 1863篇 |
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
941.
942.
Optimization theory provides a framework for determining the best decisions or actions with respect to some mathematical model of a process. This paper focuses on learning to act in a near-optimal manner through reinforcement learning for problems that either have no model or the model is too complex. One approach to solving this class of problems is via approximate dynamic programming. The application of these methods are established primarily for the case of discrete state and action spaces. In this paper we develop efficient methods of learning which act in complex systems with continuous state and action spaces. Monte-Carlo approaches are employed to estimate function values in an iterative, incremental procedure. Derivative-free line search methods are used to obtain a near-optimal action in the continuous action space for a discrete subset of the state space. This near-optimal control policy is then extended to the entire continuous state space via a fuzzy additive model. To compensate for approximation errors, a modified procedure for perturbing the generated control policy is developed. Convergence results under moderate assumptions and stopping criteria are established. 相似文献
943.
Brandau C. Bartsch T. Böhm S. Böhme C. Hoffknecht A. Kieslich S. Knopp H. Schippers S. Shi W. Müller A. Grün N. Scheid W. Steih T. Bosch F. Franzke B. Kozhuharov C. Krämer A. Mokler P. H. Nolden F. Steck M. Stöhlker T. Stachura Z. 《Hyperfine Interactions》2003,146(1-4):41-45
Hyperfine Interactions - An overview of measurements of dielectronic recombination (DR) with the heaviest lithiumlike ions is presented. The experiments have been carried out at the heavy ion... 相似文献
944.
C. Ziegler F. Baudenbacher H. Karl H. Kinder W. Göpel 《Fresenius' Journal of Analytical Chemistry》1991,341(3-4):308-313
Summary The influence of different preparation conditions and substrate surface orientations on the superconducting properties of thin YBa2Cu3O7–x (YBCO) films on silicon was studied. Comparative electrical and surface spectroscopic measurements were performed. SAM and SIMS depth profile analysis show an enrichment of barium at the interface between the superconductor and silicon for samples with Tc<76 K. Comparison with XPS data obtained for thin silicon films on YBCO indicates the formation of barium and yttrium silicates at the interface under these conditions. 相似文献
945.
In this paper we prove the existence of a solution of a coupled system involving a two phase incompressible flow in the ground and the mechanical deformation of the porous medium where the porosity is a function of the global pressure. The model is strongly coupled and involves a nonlinear degenerate parabolic equation. In order to show the existence of a weak solution, we consider a sequence of related uniformly parabolic problems and apply the Schauder fixed point theorem to show that they possess a classical solution. We then prove the relative compactness of sequences of solutions by means of the Fréchet-Kolmogorov theorem; this yields the convergence of a subsequence to a weak solution of the parabolic system. 相似文献
946.
We analyze the well-posedness of the initial value problem for the dissipative quasi-geostrophic equations in the subcritical
case. Mild solutions are obtained in several spaces with the right homogeneity to allow the existence of self-similar solutions.
While the only small self-similar solution in the strong
Lp{\cal L}^{p}
space is the null solution, infinitely many self-similar solutions do exist in weak-
Lp{\cal L}^{p}
spaces and in a recently introduced [7] space of tempered distributions. The asymptotic stability of solutions is obtained
in both spaces, and as a consequence, a criterion of self-similarity persistence at large times is obtained. 相似文献
947.
We analyse some identifiers which can univocally identify hyperbolic components and Misiurewicz points of one-dimensional quadratic maps. After seeing the equivalence among the different identifiers and how to go from one to another, we show which are the best for some specific tasks. Likewise, we present the analytic expressions, some of them shown for the first time in this paper, to calculate these identifiers. Some experimental considerations are taken into account. 相似文献
948.
Abakumov G. A. Druzhkov N. O. Kurskii Yu. A. Shavyrin A. S. 《Russian Chemical Bulletin》2003,52(3):712-717
Products of thermal transformation of substituted N-aryl-o-quinoneimines were studied using NMR spectroscopy. The formation of 4aH-phenoxazine, which was further dimerized by the Diels—Alder reaction, was established. 相似文献
949.
950.