全文获取类型
收费全文 | 22106篇 |
免费 | 1257篇 |
国内免费 | 854篇 |
专业分类
化学 | 1969篇 |
晶体学 | 71篇 |
力学 | 2336篇 |
综合类 | 200篇 |
数学 | 15621篇 |
物理学 | 4020篇 |
出版年
2024年 | 38篇 |
2023年 | 222篇 |
2022年 | 430篇 |
2021年 | 459篇 |
2020年 | 440篇 |
2019年 | 454篇 |
2018年 | 478篇 |
2017年 | 601篇 |
2016年 | 583篇 |
2015年 | 504篇 |
2014年 | 1005篇 |
2013年 | 1586篇 |
2012年 | 1120篇 |
2011年 | 1232篇 |
2010年 | 1091篇 |
2009年 | 1336篇 |
2008年 | 1411篇 |
2007年 | 1463篇 |
2006年 | 1077篇 |
2005年 | 928篇 |
2004年 | 774篇 |
2003年 | 739篇 |
2002年 | 654篇 |
2001年 | 582篇 |
2000年 | 605篇 |
1999年 | 481篇 |
1998年 | 521篇 |
1997年 | 444篇 |
1996年 | 351篇 |
1995年 | 340篇 |
1994年 | 291篇 |
1993年 | 225篇 |
1992年 | 209篇 |
1991年 | 149篇 |
1990年 | 140篇 |
1989年 | 102篇 |
1988年 | 96篇 |
1987年 | 91篇 |
1986年 | 105篇 |
1985年 | 126篇 |
1984年 | 112篇 |
1983年 | 56篇 |
1982年 | 80篇 |
1981年 | 67篇 |
1980年 | 89篇 |
1979年 | 100篇 |
1978年 | 66篇 |
1977年 | 67篇 |
1976年 | 52篇 |
1974年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Francisco‐Javier Sayas 《Numerical Methods for Partial Differential Equations》2003,19(5):555-570
This article presents and analyzes a simple method for the exterior Laplace equation through the coupling of finite and boundary element methods. The main novelty is the use of a smooth parametric artificial boundary where boundary elements fit without effort together with a straight approximate triangulation in the bounded area, with the coupling done only in nodes. A numerically integrated version of the algorithm is also analyzed. Finally, an isoparametric variant with higher order is proposed. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 555–570, 2003 相似文献
102.
Charles Blair 《Mathematical Programming》1995,71(2):127-136
We identify a class of formulas computable in polynomial time such that the functions defined by these formulas are precisely the value functions of mixed-integer programs with rational constraint coefficients. 相似文献
103.
L.E. Payne 《Journal of Mathematical Analysis and Applications》2003,284(1):283-293
We consider some initial-boundary value problems for the linear and nonlinear heat equation where the gradient of the solution is prescribed on the boundary. Assuming that a solution exists, we obtain bounds for the solution and its gradient by maximum principle arguments or by means of differential and integral inequalities. 相似文献
104.
In a previous paper we gave a new formulation and derived the Euler equations and other necessary conditions to solve strong, pathwise, stochastic variational problems with trajectories driven by Brownian motion. Thus, unlike current methods which minimize the control over deterministic functionals (the expected value), we find the control which gives the critical point solution of random functionals of a Brownian path and then, if we choose, find the expected value.This increase in information is balanced by the fact that our methods are anticipative while current methods are not. However, our methods are more directly connected to the theory and meaningful examples of deterministic variational theory and provide better means of solution for free and constrained problems. In addition, examples indicate that there are methods to obtain nonanticipative solutions from our equations although the anticipative optimal cost function has smaller expected value.In this paper we give new, efficient numerical methods to find the solution of these problems in the quadratic case. Of interest is that our numerical solution has a maximal, a priori, pointwise error of O(h3/2) where h is the node size. We believe our results are unique for any theory of stochastic control and that our methods of proof involve new and sophisticated ideas for strong solutions which extend previous deterministic results by the first author where the error was O(h2).We note that, although our solutions are given in terms of stochastic differential equations, we are not using the now standard numerical methods for stochastic differential equations. Instead we find an approximation to the critical point solution of the variational problem using relations derived from setting to zero the directional derivative of the cost functional in the direction of simple test functions.Our results are even more significant than they first appear because we can reformulate stochastic control problems or constrained calculus of variations problems in the unconstrained, stochastic calculus of variations formulation of this paper. This will allow us to find efficient and accurate numerical solutions for general constrained, stochastic optimization problems. This is not yet being done, even in the deterministic case, except by the first author. 相似文献
105.
For any Sturm-Liouville problem with a separable boundary condition and whose leading coefficient function changes sign (exactly once), we first give a geometric characterization of its eigenvalues λn using the eigenvalues of some corresponding problems with a definite leading coefficient function. Consequences of this characterization include simple proofs of the existence of the λn's, their Prüfer angle characterization, and a way for determining their indices from the zeros of their eigenfunctions. Then, interlacing relations among the λn's and the eigenvalues of the corresponding problems are obtained. Using these relations, a simple proof of asymptotic formulas for the λn's is given. 相似文献
106.
Q.J. Zhu 《Journal of Mathematical Analysis and Applications》2007,326(1):708-720
Investment systems are studied using a framework that emphasize their profiles (the cumulative probability distribution on all the possible percentage gains of trades) and their log return functions (the expected average return per trade in logarithmic scale as a function of the investment size in terms of the percentage of the available capital). The efficiency index for an investment system, defined as the maximum of the log return function, is proposed as a measure to compare investment systems for their intrinsic merit. This efficiency index can be viewed as a generalization of Shannon's information rate for a communication channel. Applications are illustrated. 相似文献
107.
We propose general variational inclusion problems which are slightly different from corresponding problems considered in several recent papers in the literature and show that they are advantageous. Sufficient conditions for the solution existence are established. As applications we derive consequences for several special cases of variational inclusion problems, quasioptimization problems, equilibrium problems and implicit variational inequalities and show that they improve the results of some recent existing papers. 相似文献
108.
首先将无线传感器网络的路由问题转化成求解最小Steiner树问题,然后给出了求解无线传感器网络路由的蚁群优化算法,并对算法的收敛性进行了证明.最后对找到最优解后信息素值的变化进行了分析.即在限制信息素取值的条件下,当迭代次数充分大时,该算法能以任意接近于1的概率找到最优解,并且当最优解找到后,最优树边上的信息素单调增加,而最优解以外边上的信息素在有限步达到最小值. 相似文献
109.
Wing-Sum Cheung Patricia J.Y. Wong 《Journal of Mathematical Analysis and Applications》2007,329(2):851-869
We consider a system of focal boundary value problems where the nonlinearities may be singular in the independent variable and may also be singular in the dependent arguments. Using Schauder fixed point theorem, we establish criteria such that the system of boundary value problems has at least one fixed-sign solution. 相似文献
110.
E. Hernández L. Rodríguez-Marín 《Journal of Optimization Theory and Applications》2007,134(1):119-134
In this paper, we study optimization problems where the objective function and the binding constraints are set-valued maps
and the solutions are defined by means of set-relations among all the images sets (Kuroiwa, D. in Takahashi, W., Tanaka, T.
(eds.) Nonlinear analysis and convex analysis, pp. 221–228, 1999). We introduce a new dual problem, establish some duality theorems and obtain a Lagrangian multiplier rule of nonlinear type
under convexity assumptions. A necessary condition and a sufficient condition for the existence of saddle points are given.
The authors thank the two referees for valuable comments and suggestions on early versions of the paper. The research of the
first author was partially supported by Ministerio de Educación y Ciencia (Spain) Project MTM2006-02629 and by Junta de Castilla
y León (Spain) Project VA027B06. 相似文献