全文获取类型
收费全文 | 4541篇 |
免费 | 475篇 |
国内免费 | 737篇 |
专业分类
化学 | 2710篇 |
晶体学 | 69篇 |
力学 | 135篇 |
综合类 | 73篇 |
数学 | 1132篇 |
物理学 | 1634篇 |
出版年
2024年 | 4篇 |
2023年 | 39篇 |
2022年 | 68篇 |
2021年 | 79篇 |
2020年 | 101篇 |
2019年 | 108篇 |
2018年 | 108篇 |
2017年 | 138篇 |
2016年 | 155篇 |
2015年 | 108篇 |
2014年 | 177篇 |
2013年 | 314篇 |
2012年 | 267篇 |
2011年 | 278篇 |
2010年 | 211篇 |
2009年 | 287篇 |
2008年 | 303篇 |
2007年 | 327篇 |
2006年 | 274篇 |
2005年 | 263篇 |
2004年 | 222篇 |
2003年 | 220篇 |
2002年 | 174篇 |
2001年 | 168篇 |
2000年 | 146篇 |
1999年 | 139篇 |
1998年 | 122篇 |
1997年 | 110篇 |
1996年 | 109篇 |
1995年 | 100篇 |
1994年 | 88篇 |
1993年 | 52篇 |
1992年 | 78篇 |
1991年 | 61篇 |
1990年 | 54篇 |
1989年 | 50篇 |
1988年 | 39篇 |
1987年 | 32篇 |
1986年 | 32篇 |
1985年 | 22篇 |
1984年 | 20篇 |
1983年 | 10篇 |
1982年 | 19篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1973年 | 6篇 |
排序方式: 共有5753条查询结果,搜索用时 31 毫秒
921.
Z.W. Yang M.Z. Liu Juan J. Nieto 《Journal of Computational and Applied Mathematics》2009,233(4):990-1004
In this paper we deal with the numerical solutions of Runge–Kutta methods for first-order periodic boundary value differential equations with piecewise constant arguments. The numerical solution is given by the numerical Green’s function. It is shown that Runge–Kutta methods preserve their original order for first-order periodic boundary value differential equations with piecewise constant arguments. We give the conditions under which the numerical solutions preserve some properties of the analytic solutions, e.g., uniqueness and comparison theorems. Finally, some experiments are given to illustrate our results. 相似文献
922.
Cristiana Bertolin 《Mathematische Zeitschrift》2009,261(4):845-868
Let S be a scheme. We compute explicitly the group of homomorphisms, the S-sheaf of homomorphisms, the group of extensions, and the S-sheaf of extensions involving locally constant S-group schemes, abelian S-schemes, and S-tori. Using the obtained results, we study the categories of biextensions involving these geometrical objects. In particular,
we prove that if G
i
(for i = 1, 2, 3) is an extension of an abelian S-scheme A
i
by an S-torus T
i
, the category of biextensions of (G
1, G
2) by G
3 is equivalent to the category of biextensions of the underlying abelian S-schemes (A
1, A
2) by the underlying S-torus T
3.
相似文献
923.
Yu. A. Chirkunov 《Siberian Mathematical Journal》2009,50(3):541-546
We show that if a system of first order linear differential equations with constant coefficients admits an operator whose coordinates responsible for the transformation of independent variables depend on functions then it admits an operator whose coordinates responsible for the transformation of functions depend nonlinearly on functions. 相似文献
924.
Estimating Turán densities of hypergraphs is believed to be one of the most challenging problems in extremal set theory. The concept of ‘jump’ concerns the distribution of Turán densities. A number α∈[0,1) is a jump for r-uniform graphs if there exists a constant c>0 such that for any family F of r-uniform graphs, if the Turán density of F is greater than α, then the Turán density of F is at least α+c. A fundamental result in extremal graph theory due to Erd?s and Stone implies that every number in [0,1) is a jump for graphs. Erd?s also showed that every number in [0,r!/rr) is a jump for r-uniform hypergraphs. Furthermore, Frankl and Rödl showed the existence of non-jumps for hypergraphs. Recently, more non-jumps were found in [r!/rr,1) for r-uniform hypergraphs. But there are still a lot of unknowns regarding jumps for hypergraphs. In this paper, we propose a new but related concept-strong-jump and describe several sequences of non-strong-jumps. It might help us to understand the distribution of Turán densities for hypergraphs better by finding more non-strong-jumps. 相似文献
925.
M. D. Kovalev 《Computational Mathematics and Mathematical Physics》2009,49(8):1369-1381
Three types of dispersion equations are analyzed that describe the eigenvalues of the effective refractive index of a multilayer plane optical waveguide and the energy eigenvalues of a quantum particle placed in a piecewise constant potential field. The first equation (D1) is derived by setting to zero the determinant of the system of linear equations produced by matching the solutions in the layers. The second equation (D2) is obtained using the well-known method of characteristic matrices. The third equation has been obtained in the general case by the author and is known as a multilayer equation. Simple relations between the three equations are established. It is shown that the set of roots of D2 exactly coincides with the set of eigenvalues of the multilayer problem, while the roots of D1 and the multilayer equation contain those equal to the refractive index in the optical case (or to the potential in the quantum case) in internal layers of the system, which may be superfluous. Examples are presented. 相似文献
926.
引入权函数,建立一个含多参量与最佳常数因子的新的反向Hilbert型积分不等式.作为应用,给出了其两个等价式及几个特殊结果. 相似文献
927.
Segmentation of three-dimensional (3D) complicated structures is of great importance for many real applications. In this work we combine graph cut minimization method with a variant of the level set idea for 3D segmentation based on the Mumford-Shah model. Compared with the traditional approach for solving the Euler-Lagrange equation we do not need to solve any partial differential equations. Instead, the minimum cut on a special designed graph need to be computed. The method is tested on data with complicated structures. It is rather stable with respect to initial value and the algorithm is nearly parameter free. Experiments show that it can solve large problems much faster than traditional approaches. 相似文献
928.
MingSheng Liu 《中国科学A辑(英文版)》2009,52(1):87-93
The Bloch constants for quasiregular harmonic mappings and open planar harmonic mappings are considered. Better estimates
are obtained. The results, presented in this paper, improve the one made by Chen et al. and Grigoryan.
This work was supported by the Research Foundation for Doctor Programme (Grant No. 20050574002) and the National Natural Science
Foundation of China (Grant No. 10471048) 相似文献
929.
Larry Guth 《Geometric And Functional Analysis》2009,18(6):1917-1987
If F is a family of mod 2 k-cycles in the unit n-ball, we lower bound the maximal volume of any cycle in F in terms of the homology class of F in the space of all cycles. We give examples to show that these lower bounds are fairly sharp.
Received: February 2007, Revision: January 2008, Accepted: January 2008 相似文献
930.
Krivdin LB Larina LI Chernyshev KA Rulev AY 《Magnetic resonance in chemistry : MRC》2006,44(2):178-187
Configurational assignment of seven azomethines obtained from the alpha-functionally substituted and nonsubstituted alpha,beta-unsaturated aldehydes has been performed on the basis of experimental measurements and the high-level ab initio calculations of their 1J(C,C) and 1J(C,H), involving the alpha-imino carbon that demonstrated the marked stereochemical dependence of both coupling constants upon the orientation of the nitrogen lone pair in the diverse isomers of the title azomethines. 相似文献