全文获取类型
收费全文 | 9787篇 |
免费 | 1097篇 |
国内免费 | 666篇 |
专业分类
化学 | 280篇 |
晶体学 | 49篇 |
力学 | 4117篇 |
综合类 | 114篇 |
数学 | 4994篇 |
物理学 | 1996篇 |
出版年
2024年 | 11篇 |
2023年 | 116篇 |
2022年 | 144篇 |
2021年 | 173篇 |
2020年 | 288篇 |
2019年 | 231篇 |
2018年 | 266篇 |
2017年 | 300篇 |
2016年 | 337篇 |
2015年 | 254篇 |
2014年 | 404篇 |
2013年 | 735篇 |
2012年 | 465篇 |
2011年 | 522篇 |
2010年 | 455篇 |
2009年 | 528篇 |
2008年 | 489篇 |
2007年 | 565篇 |
2006年 | 546篇 |
2005年 | 544篇 |
2004年 | 472篇 |
2003年 | 461篇 |
2002年 | 426篇 |
2001年 | 389篇 |
2000年 | 354篇 |
1999年 | 355篇 |
1998年 | 328篇 |
1997年 | 261篇 |
1996年 | 213篇 |
1995年 | 162篇 |
1994年 | 127篇 |
1993年 | 94篇 |
1992年 | 111篇 |
1991年 | 94篇 |
1990年 | 68篇 |
1989年 | 40篇 |
1988年 | 30篇 |
1987年 | 33篇 |
1986年 | 27篇 |
1985年 | 34篇 |
1984年 | 23篇 |
1983年 | 25篇 |
1982年 | 12篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1977年 | 4篇 |
1973年 | 2篇 |
1959年 | 2篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
101.
102.
N. D. Botkin 《Applied Mathematics and Optimization》1995,32(2):195-210
A randomized algorithm for finding a hyperplane separating two finite point sets in the Euclidean space d and a randomized algorithm for solving linearly constrained general convex quadratic problems are proposed. The expected running time of the separating algorithm isO(dd! (m + n)), wherem andn are cardinalities of sets to be separated. The expected running time of the algorithm for solving quadratic problems isO(dd! s) wheres is the number of inequality constraints. These algorithms are based on the ideas of Seidel's linear programming algorithm [6]. They are closely related to algorithms of [8], [2], and [9] and belong to an abstract class of algorithms investigated in [1]. The algorithm for solving quadratic problems has some features of the one proposed in [7].This research was done when the author was supported by the Alexander von Humboldt Foundation, Germany.On leave from the Institute of Mathematics and Mechanics, Ural Department of the Russian Academy of Sciences, 620219 Ekaterinburg, S. Kovalevskaya str. 16, Russia. 相似文献
103.
It is shown how to represent algebraically all functions that have a zero sum on all -dimensional subspaces ofPG(n,q) or ofAG(n,q). In this way one can calculate the dimensions of related codes, or one can represent interesting sets of points by functions. 相似文献
104.
Least squares estimations have been used extensively in many applications, e.g. system identification and signal prediction. When the stochastic process is stationary, the least squares estimators can be found by solving a Toeplitz or near-Toeplitz matrix system depending on the knowledge of the data statistics. In this paper, we employ the preconditioned conjugate gradient method with circulant preconditioners to solve such systems. Our proposed circulant preconditioners are derived from the spectral property of the given stationary process. In the case where the spectral density functions() of the process is known, we prove that ifs() is a positive continuous function, then the spectrum of the preconditioned system will be clustered around 1 and the method converges superlinearly. However, if the statistics of the process is unknown, then we prove that with probability 1, the spectrum of the preconditioned system is still clustered around 1 provided that large data samples are taken. For finite impulse response (FIR) system identification problems, our numerical results show that annth order least squares estimator can usually be obtained inO(n logn) operations whenO(n) data samples are used. Finally, we remark that our algorithm can be modified to suit the applications of recursive least squares computations with the proper use of sliding window method arising in signal processing applications.Research supported in part by HKRGC grant no. 221600070, ONR contract no. N00014-90-J-1695 and DOE grant no. DE-FG03-87ER25037. 相似文献
105.
Cécile Huybrechts 《Journal of Algebraic Combinatorics》1996,5(4):329-335
Let be a rank three incidence geometry of points, lines and planes whose planes are linear spaces and whose point residues are dual linear spaces (notice that we do not require anything on the line residues). We assume that the residual linear spaces of belong to a natural class of finite linear spaces, namely those linear spaces whose full automorphism group acts flag-transitively and whose orders are polynomial functions of some prime number. This class consists of six families of linear spaces. In the amalgamation of two such linear spaces imposes an equality on their orders leading, in particular, to a series of diophantine equations, the solutions of which provide a reduction theorem on the possible amalgams of linear spaces that can occur in .We prove that one of the following holds (up to a permutation of the words point and plane):A) the planes of and the dual of the point residues belong to the same family and have the same orders,B) the diagram of is in one of six families,C) the diagram of belongs to a list of seven sporadic cases.Finally, we consider the particular case where the line residues of are generalized digons. 相似文献
106.
Guido Consonni Piero Veronese 《Annals of the Institute of Statistical Mathematics》1993,45(2):303-315
Given two random variables (X, Y) the condition of unbiasedness states that:E(X |Y=y)=y andE(Y |X=x)=x both almost surely (a.s.). If the prior onY is proper and has finite expectation or non-negative support, unbiasedness impliesX=Y a.s. This paper examines the implications of unbiasedness when the prior onY is improper. Since the improper case can be meaningfully analysed in a finitely additive framework, we revisit the whole issue of unbiasedness from this perspective. First we argue that a notion weaker than equality a.s., named coincidence, is more appropriate in a finitely additive setting. Next we discuss the meaning of unbiasedness from a Bayesian and fiducial perspective. We then show that unbiasedness and finite expectation ofY imply coincidence betweenX andY, while a weaker conclusion follows if the improper prior onY is only assumed to have positive support. We illustrate our approach throughout the paper by revisiting some examples discussed in the recent literature.This work was partially supported by C.N.R. grant N.80.02970.10 (G.C.) and by C.N.R. grant altri interventi (P.V.). A preliminary draft was written while the Authors were visiting the Department of Statistics at Carnegie Mellon University.The paper is the result of close cooperation between the two authors. However subsections 3.1 and 3.3 are mainly due to G.C. while subsection 3.2 and section 4 are mainly due to P.V. 相似文献
107.
A. Bärwolff R. Puchert P. Enders U. Menzel D. Ackermann 《Journal of Thermal Analysis and Calorimetry》1995,45(3):417-436
New results of steady-state two-dimensional finite-element computations of temperature distributions of high power semiconductor laser arrays are presented. The influence of different thermal loads on the 2D temperature distribution in AlGaAs/GaAs gain-guided laser arrays is investigated. TheFEM model is tested by comparing it with analytical solutions. For numerical convenience, the latter is rewritten in a novel form, which is free of overflow problems. The maximum temperatures calculated by both methods agree within 1%. Several factors determining the thermal resistance of the device are quantitatively examined: the ratio of light emitting to non-emitting areas along the active zone, the amount of Joule losses, the current spreading, the solder thickness, and voids in the solder. This yields design rules for optimum thermal performance. 相似文献
108.
The finite field approach has been implemented in the periodic ab initio CRYSTAL program and been used for calculating the dielectric constants of crystalline LiF and MgO (FCC structure) and BeO (wurtzite structure). To maintain the periodicity along the applied field direction, a "sawtooth" potential is used in conjunction with a supercell scheme. Supercells four to five times longer than the primitive cell in the direction of the applied field provide well-converged results. The influence of the computational parameters is discussed. An alternative scheme has also been implemented, for inner check, that consists of applying a static electric field to a slab of increasing thickness in the direction orthogonal to the surface; the dielectric response at the center of the slab is shown to converge rapidly to the bulk value evaluated with the sawtooth field. The method is accurate and permits the determination of nonlinear corrections to the dielectric constant. When used in conjunction with the local density approximation (LDA) scheme, it provides for the dielectric constant of the three above-mentioned compounds values close to those recently obtained with a time-dependent density functional theory approach. 相似文献
109.
On the modelling of solid state reactions.Synthesis of YAG 总被引:2,自引:0,他引:2
Feliksas?Ivanauskas Aivaras?Kareiva Bogdanas?LapcunEmail author 《Journal of mathematical chemistry》2005,37(4):365-376
There is a model of yttrium aluminium garnet (YAG) synthesis presented in this article. The developed model is based on nonlinear reaction–diffusion partial differential equations. The solution was carried out numerically using finite difference techniques. We got dependability curves for diffusion and reaction rates and offered possible method to localize values of diffusion and reaction rate constants precisely enough.AMS subject classification: 35K57, 65M06 相似文献
110.
M. F. Webster 《Rheologica Acta》1984,23(6):582-590
There is a need to unify present hypotheses of the nature and role of the hole-pressure,p
e
, and thus provide consolidation on which to base future research and understanding. This paper is intended to meet this need. Attention is directed towards the calculation ofp
e
from the velocity and stress fields for viscoelastic fluids flowingacross rectangular holes. The constitutive models used are the Newtonian, Second-order and Maxwell models, for values of Reynolds number up to 10 and Weissenberg number up to 0.1.The numerical complications involved are studied through an investigation of the constituent parts ofp
e
. Verification of present theory is then sought, from which justification may be derived for the estimation of elasticity fromp
e
measurements. Attention is directed towards the predictions of Higashitani and Pritchard and the extension to the Tanner and Pipkin theory for Second-order fluids. The effects of variation of geometric dimensions and flow type uponp
e
are also discussed. 相似文献