全文获取类型
收费全文 | 7351篇 |
免费 | 128篇 |
国内免费 | 88篇 |
专业分类
化学 | 4527篇 |
晶体学 | 80篇 |
力学 | 157篇 |
数学 | 1712篇 |
物理学 | 1091篇 |
出版年
2022年 | 49篇 |
2020年 | 68篇 |
2019年 | 60篇 |
2018年 | 79篇 |
2017年 | 48篇 |
2016年 | 132篇 |
2015年 | 104篇 |
2014年 | 120篇 |
2013年 | 403篇 |
2012年 | 317篇 |
2011年 | 356篇 |
2010年 | 230篇 |
2009年 | 199篇 |
2008年 | 323篇 |
2007年 | 322篇 |
2006年 | 390篇 |
2005年 | 372篇 |
2004年 | 324篇 |
2003年 | 274篇 |
2002年 | 286篇 |
2001年 | 89篇 |
2000年 | 78篇 |
1999年 | 86篇 |
1998年 | 103篇 |
1997年 | 102篇 |
1996年 | 144篇 |
1995年 | 88篇 |
1994年 | 83篇 |
1993年 | 76篇 |
1992年 | 89篇 |
1991年 | 78篇 |
1990年 | 68篇 |
1989年 | 63篇 |
1988年 | 58篇 |
1987年 | 72篇 |
1986年 | 60篇 |
1985年 | 105篇 |
1984年 | 120篇 |
1983年 | 87篇 |
1982年 | 92篇 |
1981年 | 118篇 |
1980年 | 129篇 |
1979年 | 96篇 |
1978年 | 105篇 |
1977年 | 80篇 |
1976年 | 90篇 |
1975年 | 102篇 |
1974年 | 93篇 |
1973年 | 92篇 |
1972年 | 49篇 |
排序方式: 共有7567条查询结果,搜索用时 0 毫秒
21.
We consider the M(t)/M(t)/m/m queue, where the arrival rate λ(t) and service rate μ(t) are arbitrary (smooth) functions of time. Letting pn(t) be the probability that n servers are occupied at time t (0≤ n≤ m, t > 0), we study this distribution asymptotically, for m→∞ with a comparably large arrival rate λ(t) = O(m) (with μ(t) = O(1)). We use singular perturbation techniques to solve the forward equation for pn(t) asymptotically. Particular attention is paid to computing the mean number of occupied servers and the blocking probability
pm(t). The analysis involves several different space-time ranges, as well as different initial conditions (we assume that at t = 0 exactly n0 servers are occupied, 0≤ n0≤ m). Numerical studies back up the asymptotic analysis.
AMS subject classification: 60K25,34E10
Supported in part by NSF grants DMS-99-71656 and DMS-02-02815 相似文献
22.
Characterizations of real inner product spaces among a class of metric spaces have been obtained based on homogeneity of metric pythagorean orthogonality, a metrization of the concept of pythagorean orthogonality as defined in normed linear spaces. In the present paper a considerable weakening of this hypothesis is shown to characterize real inner product spaces among complete, convex, externally convex metric spaces, generalizing a result of Kapoor and Prasad [9], and providing a connection with the many characterizations of such spaces using euclidean four point properties. 相似文献
23.
Claire Cates Haulk John Drew Charles R. Johnson J. Hunter Tart 《Linear and Multilinear Algebra》1997,43(1):35-51
For a given n-by-n matrix A, we consider the set of matrices which commute with A and all of whose principal submatrices commute with the corresponding principal submatrices of A. The properties of this set are examined, with particular attention to its dimension. 相似文献
24.
We prove for a large class of tilings that, given a finite tile set, if it is possible to tile Euclideann-space with isometric copies of this set, then there is a tiling with the ‘local isomorphism property’.
Research supported in part by NSF Grant No. DMS-9001475. 相似文献
25.
The thermolysis of dicarbonylcyclopentadienyliron dimer in a sealed tube at 300°C has been reported to produce ferrocene, carbon monoxide, carbon dioxide, cyclopentadiene and elemental iron. We observe in addition to these products the production of biferrocene and elemental carbon. Thermolysis of carbonylcyclopentadienyliron tetramer (produced from the dimer by thermolysis in refluxing xylene) produces ferrocene, iron and carbon monoxidre; no biferrocene, carbon dioxide or elemental carbon are produced. A scheme is proposed to account for these products. 相似文献
26.
27.
N-7, O-Disilylation of 4(3H)-oxo-7H-pyrrolo[2,3-d]pyrimidines provides a convenient method for activation of these compounds toward regiospecific halogenation at C-5. The sequence is conveniently carried out without isolation of the silylated derivatives and desilylation is spontaneous upon addition of water to the reaction mixture. 相似文献
28.
Alick KT Law Deepak Gupta Shawn Levy Douglas C Wallace Robert J McKeon Charles R Buck 《BMC neuroscience》2004,5(1):1
Background
The adenine nucleotide translocator 1 (Ant1) is an inner mitochondrial membrane protein involved with energy mobilization during oxidative phosphorylation. We recently showed that rodent Ant1 is upregulated by transforming growth factor-beta (TGF-β) in reactive astrocytes following CNS injury. In the present study, we describe the molecular mechanisms by which TGF-β1 regulates Ant1 gene expression in cultured primary rodent astrocytes. 相似文献29.
Jean-Philippe Bouillon Charles Portella Alessandro Degl’Innocenti 《Tetrahedron letters》2004,45(1):87-90
The thionation of bis(acylsilanes) with spacers of variable size with hexamethyldisilathiane under cobalt(II) chloride or trimethylsilyl triflate catalysis affords 2,5-bis(trialkylsilyl)-thiophenes, 2,6-bis(trialkylsilyl)-4H-thiopyrans and 2,7-bis(trialkylsilyl)-4,5-dihydrothiepine generally along with a minor amount of the corresponding oxo analogue. The synthesis of both symmetrical and unsymmetrical bis(trialkylsilyl) derivatives was achieved. 相似文献
30.
The passive control of bluff body flows using porous media is investigated by means of the penalization method. This method is used to create intermediate porous media between solid obstacles and the fluid in order to modify the boundary layer behaviour. The study covers a wide range of two‐dimensional flows from low transitional flow to fully established turbulence by direct numerical simulation of incompressible Navier–Stokes equations. A parametric study is performed to illustrate the effect of the porous layer permeability and thickness on the passive control. The numerical results reveal the ability of porous media to both regularize the flow and to reduce the drag forces up to 30%. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献