全文获取类型
收费全文 | 4935篇 |
免费 | 171篇 |
国内免费 | 126篇 |
专业分类
化学 | 124篇 |
晶体学 | 5篇 |
力学 | 1544篇 |
综合类 | 3篇 |
数学 | 2579篇 |
物理学 | 977篇 |
出版年
2024年 | 6篇 |
2023年 | 71篇 |
2022年 | 85篇 |
2021年 | 83篇 |
2020年 | 147篇 |
2019年 | 83篇 |
2018年 | 93篇 |
2017年 | 116篇 |
2016年 | 135篇 |
2015年 | 145篇 |
2014年 | 290篇 |
2013年 | 532篇 |
2012年 | 154篇 |
2011年 | 348篇 |
2010年 | 294篇 |
2009年 | 370篇 |
2008年 | 324篇 |
2007年 | 296篇 |
2006年 | 234篇 |
2005年 | 157篇 |
2004年 | 167篇 |
2003年 | 136篇 |
2002年 | 112篇 |
2001年 | 85篇 |
2000年 | 57篇 |
1999年 | 67篇 |
1998年 | 78篇 |
1997年 | 51篇 |
1996年 | 42篇 |
1995年 | 32篇 |
1994年 | 45篇 |
1993年 | 49篇 |
1992年 | 52篇 |
1991年 | 52篇 |
1990年 | 43篇 |
1989年 | 32篇 |
1988年 | 22篇 |
1987年 | 33篇 |
1986年 | 12篇 |
1985年 | 32篇 |
1984年 | 29篇 |
1983年 | 18篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1980年 | 1篇 |
1978年 | 2篇 |
排序方式: 共有5232条查询结果,搜索用时 0 毫秒
41.
Dessislava H. Kochloukova 《代数通讯》2013,41(3):785-796
We define and study the property finite presentability in the category of Hopf algebras that are smash product of universal enveloping algebra of a Lie algebra by a group algebra. We show that for such Hopf algebras finite presentability is equivalent with finite presentability as an associative k-algebra. 相似文献
42.
43.
Mingxiang Chen Dominic P. Clemence 《Journal of Difference Equations and Applications》2013,19(9):887-899
This paper considers a non-linear system of ordinary differential equations, which arises in the study of hantavirus epidemics. The system has the property that the total population obeys the logistic equation. For this system, we use linearization and the dynamical properties of the logistic equation to analyze the dynamics of the subpopulation system. In view of the usual numerical instabilities associated with standard finite difference methods used for simulating such systems, we construct non-standard finite difference (NSFD) schemes, which preserve the dynamic properties of the system, and may therefore be used for its simulation. 相似文献
44.
Marcus R. Garvie Philip K. Maini Catalin Trenchea 《Journal of computational physics》2010,229(19):7058-7071
We present a new algorithm for estimating parameters in reaction–diffusion systems that display pattern formation via the mechanism of diffusion-driven instability. A Modified Discrete Optimal Control Algorithm (MDOCA) is illustrated with the Schnakenberg and Gierer–Meinhardt reaction–diffusion systems using PDE constrained optimization techniques. The MDOCA algorithm is a modification of a standard variable step gradient algorithm that yields a huge saving in computational cost. The results of numerical experiments demonstrate that the algorithm accurately estimated key parameters associated with stationary target functions generated from the models themselves. Furthermore, the robustness of the algorithm was verified by performing experiments with target functions perturbed with various levels of additive noise. The MDOCA algorithm could have important applications in the mathematical modeling of realistic Turing systems when experimental data are available. 相似文献
45.
46.
Shadow systems are often used to approximate reaction-diffusion systems when one of the diffusion rates is large. In this paper, we study the global existence and blow-up phenomena for shadow systems. Our results show that even for these fundamental aspects, there are serious discrepancies between the dynamics of the reaction-diffusion systems and that of their corresponding shadow systems. 相似文献
47.
Quasi-interpolation is an important tool, used both in theory and in practice, for the approximation of smooth functions from univariate or multivariate spaces which contain , the d-variate polynomials of degree ≤m. In particular, the reproduction of Πm leads to an approximation order of m+1. Prominent examples include Lagrange and Bernstein type approximations by polynomials, the orthogonal projection onto Πm for some inner product, finite element methods of precision m, and multivariate spline approximations based on macroelements or the translates of a single spline.For such a quasi-interpolation operator L which reproduces and any r≥0, we give an explicit construction of a quasi-interpolant which reproduces Πm+r, together with an integral error formula which involves only the (m+r+1)th derivative of the function approximated. The operator is defined on functions with r additional orders of smoothness than those on which L is defined. This very general construction holds in all dimensions d. A number of representative examples are considered. 相似文献
48.
A coefficient inverse problem of the one-dimensional parabolic equation is solved by a high-order compact finite difference method in this paper. The problem of recovering a time-dependent coefficient in a parabolic partial differential equation has attracted considerable attention recently. While many theoretical results regarding the existence and uniqueness of the solution are obtained, the development of efficient and accurate numerical methods is still far from satisfactory. In this paper a fourth-order efficient numerical method is proposed to calculate the function u(x,t) and the unknown coefficient a(t) in a parabolic partial differential equation. Several numerical examples are presented to demonstrate the efficiency and accuracy of the numerical method. 相似文献
49.
A Galerkin finite element method is developed for the two dimensional/three dimensional nonlinear time-dependent three-species Lotka–Volterra competition-diffusion equations on a bounded domain. The existence and uniqueness of the solution to the numerical formulation are proved. An error estimate for the numerical solution is obtained. Numerical computations are carried out to examine the expected orders of accuracy in the error estimates. 相似文献
50.
The structures of 1,3-substituted squaraine derivatives Sq1 ~ Sq12 were fully optimized by ab initio HF method with 6-31G* basis set level,and the electronic structures of Sq1 ~ Sq12 were also calculated. Based on the optimized structures,the electronic spectra were obtained by the CIS / 6-31G* method,which suggested that the max absorption wavelength mainly resulted from the electronic transition from HOMO to LUMO. The second nonlinear optical coefficients(β0)were calculated using ab initio CPHF method at 6-31G* basis set level and FF / AM1,FF / PM3,FF / MNDO,FF / MINDO3 methods. A systematic comparison between the results was carried out. It indicated that the second nonlinear optical coefficients were affected dramatically by the properties of five-membered hetero-rings. β0 can be enhanced by introducing pyrrole,thiazole and oxazole. The position of five-membered rings containing two heteroatoms which were connected with four-membered squaraine rings also affected β0 . 相似文献