全文获取类型
收费全文 | 453656篇 |
免费 | 3057篇 |
国内免费 | 1399篇 |
专业分类
化学 | 223756篇 |
晶体学 | 6630篇 |
力学 | 24024篇 |
综合类 | 16篇 |
数学 | 74114篇 |
物理学 | 129572篇 |
出版年
2020年 | 3497篇 |
2019年 | 4179篇 |
2018年 | 15852篇 |
2017年 | 15647篇 |
2016年 | 13672篇 |
2015年 | 5017篇 |
2014年 | 7451篇 |
2013年 | 16484篇 |
2012年 | 15911篇 |
2011年 | 24751篇 |
2010年 | 16530篇 |
2009年 | 16962篇 |
2008年 | 20478篇 |
2007年 | 22383篇 |
2006年 | 13036篇 |
2005年 | 12784篇 |
2004年 | 12067篇 |
2003年 | 11588篇 |
2002年 | 10507篇 |
2001年 | 10619篇 |
2000年 | 8315篇 |
1999年 | 6196篇 |
1998年 | 5381篇 |
1997年 | 5457篇 |
1996年 | 5109篇 |
1995年 | 4315篇 |
1994年 | 4472篇 |
1993年 | 4435篇 |
1992年 | 4811篇 |
1991年 | 4950篇 |
1990年 | 4732篇 |
1989年 | 4794篇 |
1988年 | 4656篇 |
1987年 | 4627篇 |
1986年 | 4431篇 |
1985年 | 5689篇 |
1984年 | 5863篇 |
1983年 | 4797篇 |
1982年 | 5062篇 |
1981年 | 4916篇 |
1980年 | 4538篇 |
1979年 | 5081篇 |
1978年 | 5171篇 |
1977年 | 5310篇 |
1976年 | 5326篇 |
1975年 | 4984篇 |
1974年 | 4896篇 |
1973年 | 5133篇 |
1972年 | 3683篇 |
1971年 | 3348篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
861.
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
862.
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
863.
Inhibition of the reduction of Cr(VI) at the magnetite–water interface by calcium carbonate coatings
The effect of calcium carbonate coatings on the reduction of aqueous chromate on the magnetite(1 1 1) surface has been investigated using a combination of synchrotron based X-ray photoemission spectroscopy (PES) and X-ray absorption near edge structure (XANES) spectroscopy, along with laboratory-based powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). CaCO3 coatings (dominantly calcite with minor quantities of aragonite and vaterite) of thicknesses ranging from 10 Å to 20 m were grown on magnetite(1 1 1) surfaces by exposure to supersaturated aqueous solutions followed by evaporation of the solution—a process that mimics pore-water evaporation in vadose zones leading to the formation of caliche and calcium carbonate coatings on mineral grains. Coating thicknesses were determined from attenuation of the Fe 2p photoemission signal by the carbonate coating. For coatings less than 15 Å thick, Cr 2p photoemission and Cr LII, LIII-edge XANES spectra show that chromate is reduced by the underlying magnetite surface; however, as the minimum coating thickness increases beyond 15 Å, the magnetite surface becomes passivated and further chromate reduction ceases. Our findings suggest that carbonate coatings on natural magnetite grains can significantly reduce or eliminate their ability to reduce Cr(VI), which is a toxic and highly mobile environmental contaminant. 相似文献
864.
We develop a lattice-dynamical model based on the screw symmetry of single-walled carbon nanotubes that allows for reducing the size of the dynamical matrix to six, for all tube chiralities. The model uses force constants derived from fitting to the phonon dispersion of 2D graphite. We present the calculation procedure in a clear and transparent way, making the model easier to follow. We calculate the phonon dispersions of a number of nanotubes of different chiralities. The splitting of two highest Raman active modes and the radial breathing mode frequency are studied by changing the tube diameter and chirality. 相似文献
865.
Serge Tabachnikov 《Geometriae Dedicata》2007,129(1):83-87
We prove a recent theorem by C. Culter every polygonal outer billiard in the affine plane has a periodic trajectory.
相似文献
866.
Most of the few known examples of compact Riemannian manifolds with positive sectional curvature are the total space of a
Riemannian submersion. In this article we show that this is true for all known examples, if we enlarge the category to orbifold
fibrations. For this purpose we study all almost free isometric circle actions on positively curved Eschenburg spaces, which
give rise to principle orbifold bundle structures, and we examine in detail their geometric properties. In particular, we
obtain a new family of 6-dimensional orbifolds with positive sectional curvature whose singular locus consists of just two
points.
相似文献
867.
The paper proves the existence of solutions to the magneto-hydrodynamics equations driven by random exterior forcing terms
both in the velocity and in the magnetic field. The existence and uniqueness of an invariant measure is also obtained via
coupling methods. 相似文献
868.
869.
870.