全文获取类型
收费全文 | 755132篇 |
免费 | 7073篇 |
国内免费 | 2214篇 |
专业分类
化学 | 392639篇 |
晶体学 | 10726篇 |
力学 | 35921篇 |
综合类 | 23篇 |
数学 | 98330篇 |
物理学 | 226780篇 |
出版年
2021年 | 6297篇 |
2020年 | 6923篇 |
2019年 | 7653篇 |
2018年 | 9861篇 |
2017年 | 9901篇 |
2016年 | 14441篇 |
2015年 | 8617篇 |
2014年 | 13717篇 |
2013年 | 33803篇 |
2012年 | 25902篇 |
2011年 | 31324篇 |
2010年 | 22360篇 |
2009年 | 22196篇 |
2008年 | 29277篇 |
2007年 | 29273篇 |
2006年 | 27167篇 |
2005年 | 24305篇 |
2004年 | 22443篇 |
2003年 | 20074篇 |
2002年 | 19919篇 |
2001年 | 21339篇 |
2000年 | 16512篇 |
1999年 | 12959篇 |
1998年 | 10990篇 |
1997年 | 10734篇 |
1996年 | 10340篇 |
1995年 | 9349篇 |
1994年 | 9156篇 |
1993年 | 8846篇 |
1992年 | 9561篇 |
1991年 | 9930篇 |
1990年 | 9439篇 |
1989年 | 9242篇 |
1988年 | 9277篇 |
1987年 | 8885篇 |
1986年 | 8508篇 |
1985年 | 11270篇 |
1984年 | 11667篇 |
1983年 | 9756篇 |
1982年 | 10374篇 |
1981年 | 9935篇 |
1980年 | 9467篇 |
1979年 | 10059篇 |
1978年 | 10321篇 |
1977年 | 10218篇 |
1976年 | 10144篇 |
1975年 | 9714篇 |
1974年 | 9470篇 |
1973年 | 9888篇 |
1972年 | 6742篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
It is pointed out that the observation of the electrostatic ion acoustic wave frequency can be a suitable check to determine whether the produced plasma is a pure pair-ion plasma or whether it comprises some concentration of electrons. A theoretical model for the pair-ion plasma dynamics is presented along with a new electrostatic mode which can exist only in such systems. It can become unstable in the presence of shear flow and it can give rise to vortex structures in the nonlinear regime. The possibility of shocks and solitons, due to nonlinear drift waves in a pair-ion plasma comprising electrons, is also discussed. The relevance of this investigation to both laboratory and astrophysical plasmas is pointed out. 相似文献
992.
We prove a conjecture by F. Ferrari. Let X be the total space of a nonlinear deformation of a rank two holomorphic vector bundle on a smooth rational curve, such that X has trivial canonical bundle and has sections. Then the normal bundle to such sections is computed in terms of the rank of the Hessian of a suitably defined superpotential at its critical points. 相似文献
993.
For fairly high concentrations, the effective arsenic diffusioncoefficient is known to grow linearly with the concentration.In this paper, we consider the strongly enhanced diffusion ofarsenic by asymptotic methods, and extend the results of a previouspaper (King & Please, 1987) in three ways: we characterizean additional (low-concentration) region; we consider the influenceof the initial data; and we consider the case of constant surfaceconcentration. 相似文献
994.
J. Palis 《Publications Mathématiques de L'IHéS》1987,66(1):211-215
995.
We present theoretical results of the electron impact ionization rate in GaAs/AlGaAs multiquantum well structures as a function of applied electric field for various geometries, i.e., well and barrier widths. In addition, we present preliminary measurements of the current-voltage characteristics of MBE grown devices which demonstrate very low leakage current as well as sharp breakdwon behavior. It is found that the net ionization rate, determined by averaging over the constitutent GaAs and AlGaAs layers, approaches the weighted average of the constituent bulk rates at high electric field strengths; the potential discontinuity is relatively unimportant. The electron ionization rate within the well regions alone is still higher than that in bulk GaAs, but is insufficiently enhanced to compensate for the much lower rate in the AlGaAs layers. As the field is lowered to 250.0 kV/cm, the average ionization rate in the multiquantum well structure becomes larger than in the bulk. 相似文献
996.
We present a framework for extending Stone's representation theorem for distributive lattices to representation theorems for distributive lattices with operators. We proceed by introducing the definition of algebraic theory of operators over distributive lattices. Each such theory induces a functor on the category of distributive lattices such that its algebras are exactly the distributive lattices with operators in the original theory. We characterize the topological counterpart of these algebras in terms of suitable coalgebras on spectral spaces. We work out some of these coalgebraic representations, including a new representation theorem for distributive lattices with monotone operators. 相似文献
997.
We investigate separability questions for the mapping class group of a surface. While this group is not subgroup separable in general, we prove a large family of interesting subgroups are separable. This includes many classically studied subgroups such as solvable subgroups, Heegaard and Handlebody groups, geometric subgroups, and all the terms in the Johnson filtration. 相似文献
998.
N C Proudlove S Black A Fletcher 《The Journal of the Operational Research Society》2007,58(2):145-158
This paper considers efforts to improve in-patient flows, a particularly urgent issue in the National Health Service (NHS). The context is described and related to reasons why OR has been making relatively little contribution. The paper argues that large complex models may often be unnecessary and even get in the way of providing clear insight and guidance for problem owners. The importance of understanding the generic working of systems to lead to improvement, and the limitations of simply describing them, is stressed. It is demonstrated that some very simple models can be of significant practical value in understanding and managing complex systems, changing mindsets and driving collection and use of operationally valuable data. Recommendations for more effective engagement with the NHS are offered. 相似文献
999.
1000.
We give counterexamples to the following conjecture of Auslander: given a finitely generated module M over an Artin algebra Λ, there exists a positive integer nM such that for all finitely generated Λ-modules N, if ExtΛi(M,N)=0 for all i?0, then ExtΛi(M,N)=0 for all i?nM. Some of our examples moreover yield homologically defined classes of commutative local rings strictly between the class of local complete intersections and the class of local Gorenstein rings. 相似文献