全文获取类型
收费全文 | 13957篇 |
免费 | 1749篇 |
国内免费 | 488篇 |
专业分类
化学 | 5476篇 |
晶体学 | 33篇 |
力学 | 1500篇 |
综合类 | 86篇 |
数学 | 5321篇 |
物理学 | 3778篇 |
出版年
2023年 | 155篇 |
2022年 | 172篇 |
2021年 | 245篇 |
2020年 | 439篇 |
2019年 | 331篇 |
2018年 | 308篇 |
2017年 | 348篇 |
2016年 | 592篇 |
2015年 | 555篇 |
2014年 | 784篇 |
2013年 | 1121篇 |
2012年 | 793篇 |
2011年 | 860篇 |
2010年 | 609篇 |
2009年 | 881篇 |
2008年 | 845篇 |
2007年 | 885篇 |
2006年 | 765篇 |
2005年 | 620篇 |
2004年 | 549篇 |
2003年 | 528篇 |
2002年 | 441篇 |
2001年 | 426篇 |
2000年 | 403篇 |
1999年 | 331篇 |
1998年 | 328篇 |
1997年 | 258篇 |
1996年 | 255篇 |
1995年 | 195篇 |
1994年 | 146篇 |
1993年 | 133篇 |
1992年 | 93篇 |
1991年 | 94篇 |
1990年 | 78篇 |
1989年 | 61篇 |
1988年 | 63篇 |
1987年 | 45篇 |
1986年 | 56篇 |
1985年 | 35篇 |
1984年 | 50篇 |
1983年 | 18篇 |
1982年 | 27篇 |
1981年 | 30篇 |
1980年 | 21篇 |
1979年 | 25篇 |
1978年 | 44篇 |
1977年 | 42篇 |
1976年 | 41篇 |
1975年 | 16篇 |
1974年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 875 毫秒
81.
82.
Jean-Pierre Dedieu Gregorio Malajovich Mike Shub 《Foundations of Computational Mathematics》2005,5(2):145-171
We prove a linear bound on the average total curvature of the
central path of linear programming theory in terms of the number
of independent variables of the primal problem, and independent of
the number of constraints. 相似文献
83.
Numerical schemes for systems with multiple spatio-temporal scales are investigated. The multiscale schemes use asymptotic results for this type of systems which guarantee the existence of an effective dynamics for some suitably defined modes varying slowly on the largest scales. The multiscale schemes are analyzed in general, then illustrated on a specific example of a moderately large deterministic system displaying chaotic behavior due to Lorenz. Issues like consistency, accuracy, and efficiency are discussed in detail. The role of possible hidden slow variables as well as additional effects arising on the diffusive time-scale are also investigated. As a byproduct we obtain a rather complete characterization of the effective dynamics in Lorenz model. 相似文献
84.
Standard ODE methods such as linear multistep methods encounter difficulties when applied to differential-algebraic equations (DAEs) of index greater than 1. In particular, previous results for index 2 DAEs have practically ruled out the use of all explicit methods and of implicit multistep methods other than backward difference formulas (BDFs) because of stability considerations. In this paper we embed known results for semi-explicit index 1 and 2 DAEs in a more comprehensive theory based on compound multistep and one-leg discretizations. This explains and characterizes the necessary requirements that a method must fulfill in order to be applicable to semi-explicit DAEs. Thus we conclude that the most useful discretizations are those that avoid discretization of the constraint. A freer use of e.g. explicit methods for the non-stiff differential part of the DAE is then possible.Dedicated to Germund Dahlquist on the occasion of his 70th birthdayThis author thanks the Centro de Estadística y Software Matemático de la Universidad Simón Bolivar (CESMa) for permitting her free use of its research facilities.Partial support by the Swedish Research Council for Engineering Sciences TFR under contract no. 222/91-405. 相似文献
85.
S. Al-Homidan 《Journal of Optimization Theory and Applications》2007,135(3):583-598
Given a data matrix, we find its nearest symmetric positive-semidefinite Toeplitz matrix. In this paper, we formulate the
problem as an optimization problem with a quadratic objective function and semidefinite constraints. In particular, instead
of solving the so-called normal equations, our algorithm eliminates the linear feasibility equations from the start to maintain
exact primal and dual feasibility during the course of the algorithm. Subsequently, the search direction is found using an
inexact Gauss-Newton method rather than a Newton method on a symmetrized system and is computed using a diagonal preconditioned
conjugate-gradient-type method. Computational results illustrate the robustness of the algorithm. 相似文献
86.
Asymptotic methods for contact problems are expounded. Some typical integral equations are considered 相似文献
87.
A formulation of an implicit characteristic-flux-averaging method for the unsteady Euler equations with real gas effects is presented. Incorporation of a real gas into a general equation of state is achieved by considering the pressure as a function of density and specific internal energy. The Ricmann solver as well as the flux-split algorithm are modified by introducing the pressure derivatives with respect to density and internal energy. Expressions for calculating the values of the flow variables for a real gas at the cell faces are derived. The Jacobian matrices and the eigenvectors are defined for a general equation of state. The solution of the system of equations is obtained by using a mesh-sequencing method for acceleration of the convergence. Finally, a test case for a simple form of equation of state displays the differences from the corresponding solution for an ideal gas. 相似文献
88.
The theoretical investigation of the image states in front of an ultrathin iron film grown on copper has been performed by means of the embedding method and a recently developed procedure for the inclusion of the image potential tail in a first principle calculation. From the electronic response to an applied electric field, the image plane position has been evaluated. This also allows one to obtain useful information about the spin dependent screening properties of the system. Exchange splitting, effective mass, and lifetime of such surface states result in good agreement with recently performed two-photon photoemission experiments [see A.B. Schmidt, M. Pickel, M. Wiemhöfer, M. Donath, M. Weinelt, Phys. Rev. Lett. 95 (2005) 107402]. 相似文献
89.
四种多变量校准方法在FTIR多组分分析中的性能比较 总被引:1,自引:0,他引:1
本文对四种多变量校准方法--经典最小二乘法(CLS),偏最小二乘法(PLS),卡尔曼滤波法(KFM)以及人工神经网络法(ANN)--在多组分浓度分析方面的性能进行了比较。选择五种红外谱图严重混叠的大气有机毒物--1,3-丁二烯,苯,邻二甲苯,氯苯和丙烯醛--作为分析对象。分别计算各种方法对该5组分体系的平均预测误差MPE和平均相对误差MRE进行比较。结果表明,偏最小二乘法在处理这类问题中是最稳健的方法。 相似文献
90.
C. Bogani M. G. Gasparo A. Papini 《Journal of Optimization Theory and Applications》2007,134(1):47-59
We propose a pattern search method to solve a classical nonsmooth optimization problem. In a deep analogy with pattern search
methods for linear constrained optimization, the set of search directions at each iteration is defined in such a way that
it conforms to the local geometry of the set of points of nondifferentiability near the current iterate. This is crucial to
ensure convergence. The approach presented here can be extended to wider classes of nonsmooth optimization problems. Numerical
experiments seem to be encouraging.
This work was supported by M.U.R.S.T., Rome, Italy. 相似文献