全文获取类型
收费全文 | 26528篇 |
免费 | 3054篇 |
国内免费 | 1410篇 |
专业分类
化学 | 1457篇 |
晶体学 | 152篇 |
力学 | 4416篇 |
综合类 | 259篇 |
数学 | 18071篇 |
物理学 | 6637篇 |
出版年
2024年 | 53篇 |
2023年 | 270篇 |
2022年 | 301篇 |
2021年 | 407篇 |
2020年 | 695篇 |
2019年 | 689篇 |
2018年 | 657篇 |
2017年 | 736篇 |
2016年 | 771篇 |
2015年 | 626篇 |
2014年 | 1123篇 |
2013年 | 2460篇 |
2012年 | 1128篇 |
2011年 | 1446篇 |
2010年 | 1230篇 |
2009年 | 1592篇 |
2008年 | 1650篇 |
2007年 | 1650篇 |
2006年 | 1533篇 |
2005年 | 1313篇 |
2004年 | 1182篇 |
2003年 | 1199篇 |
2002年 | 1108篇 |
2001年 | 918篇 |
2000年 | 911篇 |
1999年 | 827篇 |
1998年 | 664篇 |
1997年 | 623篇 |
1996年 | 447篇 |
1995年 | 347篇 |
1994年 | 297篇 |
1993年 | 277篇 |
1992年 | 213篇 |
1991年 | 180篇 |
1990年 | 155篇 |
1989年 | 106篇 |
1988年 | 100篇 |
1987年 | 95篇 |
1986年 | 75篇 |
1985年 | 115篇 |
1984年 | 140篇 |
1983年 | 68篇 |
1982年 | 93篇 |
1981年 | 102篇 |
1980年 | 80篇 |
1979年 | 78篇 |
1978年 | 87篇 |
1977年 | 60篇 |
1976年 | 42篇 |
1974年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
31.
A simple scheme is developed for treatment of vertical bed topography in shallow water flows. The effect of the vertical step on flows is modelled with the shallow water equations including local energy loss terms. The bed elevation is denoted with zb‐ for the left and zb+ for the right values at each grid point, hence exactly representing a discontinuity in the bed topography. The surface gradient method (SGM) is generalized to reconstruct water depths at cell interfaces involving a vertical step so that the fluxes at the cell interfaces can accurately be calculated with a Riemann solver. The scheme is verified by predicting a surge crossing a step, a tidal flow over a step and dam‐break flows on wet/dry beds. The results have shown good agreements compared with analytical solutions and available experimental data. The scheme is efficient, robust, and may be used for practical flow calculations. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
32.
33.
A. V. Bobylev 《Journal of statistical physics》2006,124(2-4):371-399
It is well-known that the classical Chapman-Enskog procedure does not work at the level of Burnett equations (the next step after the Navier-Stokes equations). Roughly speaking, the reason is that the solutions of higher equations of hydrodynamics (Burnett's, etc.) become unstable with respect to short-wave perturbations. This problem was recently attacked by several authors who proposed different ways to deal with it. We present in this paper one of possible alternatives. First we deduce a criterion for hyperbolicity of Burnett equations for the general molecular model and show that this criterion is not fulfilled in most typical cases. Then we discuss in more detail the problem of truncation of the Chapman-Enskog expansion and show that the way of truncation is not unique. The general idea of changes of coordinates (based on analogy with the theory of dynamical systems) leads finally to nonlinear Hyperbolic Burnett Equations (HBEs) without using any information beyond the classical Burnett equations. It is proved that HBEs satisfy the linearized H-theorem. The linear version of the problem is studied in more detail, the complete Chapman-Enskog expansion is given for the linear case. A simplified proof of the Slemrod identity for Burnett coefficients is also given. 相似文献
34.
35.
Cai Xiaochun 《Annals of Differential Equations》2006,22(1):1-6
In this paper, we apply a critical point theorem and obtain the condition for the existence of three solutions to two-point boundary value problem of a second order nonlinear difference equation. 相似文献
36.
The molding processes of polymer melts involve geometrically complex dies. Such dies are usually tapered or streamlined to achieve a maximum output rate under conditions of laminar flow. The model of a generalized second-grade fluid of power-law type is used and the results obtained are illustrated by examples of convergent flows in conical and wedge-shaped dies. 相似文献
37.
This paper is concerned with the implementation and testing of an algorithm for solving constrained least-squares problems. The algorithm is an adaptation to the least-squares case of sequential quadratic programming (SQP) trust-region methods for solving general constrained optimization problems. At each iteration, our local quadratic subproblem includes the use of the Gauss–Newton approximation but also encompasses a structured secant approximation along with tests of when to use this approximation. This method has been tested on a selection of standard problems. The results indicate that, for least-squares problems, the approach taken here is a viable alternative to standard general optimization methods such as the Byrd–Omojokun trust-region method and the Powell damped BFGS line search method. 相似文献
38.
Johannes Elschner Rainer Hinder Gunther Schmidt 《Advances in Computational Mathematics》2002,16(2-3):139-156
This paper is devoted to the numerical study of diffraction by periodic structures of plane waves under oblique incidence. For this situation Maxwell's equations can be reduced to a system of two Helmholtz equations in R
2 coupled via quasiperiodic transmission conditions on the piecewise smooth interfaces between different materials. The numerical analysis is based on a strongly elliptic variational formulation of the differential problem in a bounded periodic cell involving nonlocal boundary operators. We obtain existence and uniqueness results for discrete solutions and provide the corresponding error analysis. 相似文献
39.
Summary Considered here are model equations for weakly nonlinear and dispersive long waves, which feature general forms of dispersion and pure power nonlinearity. Two variants of such equations are introduced, one of Korteweg-de Vries type and one of regularized long-wave type. It is proven that solutions of the pure initial-value problem for these two types of model equations are the same, to within the order of accuracy attributable to either, on the long time scale during which nonlinear and dispersive effects may accumulate to make an order-one relative difference to the wave profiles.This research was supported in part by the National Science Foundation. A considerable portion of the project was completed while the first author was resident at the Institute for Mathematics and Its Applications, University of Minnesota. 相似文献
40.