全文获取类型
收费全文 | 93676篇 |
免费 | 8210篇 |
国内免费 | 9171篇 |
专业分类
化学 | 32120篇 |
晶体学 | 404篇 |
力学 | 8895篇 |
综合类 | 1441篇 |
数学 | 36598篇 |
物理学 | 31599篇 |
出版年
2024年 | 148篇 |
2023年 | 872篇 |
2022年 | 1615篇 |
2021年 | 1827篇 |
2020年 | 2108篇 |
2019年 | 2314篇 |
2018年 | 1971篇 |
2017年 | 2270篇 |
2016年 | 2503篇 |
2015年 | 2372篇 |
2014年 | 3499篇 |
2013年 | 6834篇 |
2012年 | 3795篇 |
2011年 | 4641篇 |
2010年 | 3940篇 |
2009年 | 5379篇 |
2008年 | 5843篇 |
2007年 | 5940篇 |
2006年 | 5540篇 |
2005年 | 4741篇 |
2004年 | 4231篇 |
2003年 | 4310篇 |
2002年 | 4031篇 |
2001年 | 3382篇 |
2000年 | 3290篇 |
1999年 | 2933篇 |
1998年 | 2653篇 |
1997年 | 2202篇 |
1996年 | 1940篇 |
1995年 | 1691篇 |
1994年 | 1572篇 |
1993年 | 1307篇 |
1992年 | 1267篇 |
1991年 | 971篇 |
1990年 | 785篇 |
1989年 | 705篇 |
1988年 | 613篇 |
1987年 | 500篇 |
1986年 | 419篇 |
1985年 | 525篇 |
1984年 | 538篇 |
1983年 | 266篇 |
1982年 | 418篇 |
1981年 | 463篇 |
1980年 | 353篇 |
1979年 | 352篇 |
1978年 | 277篇 |
1977年 | 240篇 |
1976年 | 186篇 |
1974年 | 122篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
In this work we study nonnegativity and positivity of a discrete quadratic functional with separately varying endpoints. We introduce a notion of an interval coupled with 0, and hence, extend the notion of conjugate interval to 0 from the case of fixed to variable endpoint(s). We show that the nonnegativity of the discrete quadratic functional is equivalent to each of the following conditions: The nonexistence of intervals coupled with 0, the existence of a solution to Riccati matrix equation and its boundary conditions. Natural strengthening of each of these conditions yields a characterization of the positivity of the discrete quadratic functional. Since the quadratic functional under consideration could be a second variation of a discrete calculus of variations problem with varying endpoints, we apply our results to obtain necessary and sufficient optimality conditions for such problems. This paper generalizes our recent work in [R. Hilscher, V. Zeidan, Comput. Math. Appl., to appear], where the right endpoint is fixed. 相似文献
32.
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
33.
34.
35.
不完全信息群体决策专家权重的集结 总被引:8,自引:5,他引:3
本文对于属性权重信息和属性效用信息都不完全的群体多属性决策问题,通过构造属性值区间和运用系统聚类分析法,对群体决策中的专家进行分类,并确定每位专家的权重. 相似文献
36.
通过分析计算,设计了一种大口径宽视场折射式红外相机光学系统。该系统设计用于超长线阵扫描红外相机的地面演示成像,它能同时对短波2μm—3μm、中波3μm—4.5μm双波段成像,且同时具有24°的大视场和150mm的宽口径,成像质量接近衍射极限。该系统在航天遥感领域应用广泛。 相似文献
37.
An Nd:YAG laser-pumped periodically poled RbTiOAsO4 (PPRTA) optical parametric oscillator (OPO) near degeneracy is reported. At a pump power of 3.7 W, a net conversion efficiency of 43% was obtained. These crystals were found to be phase-matchable for the higher order quasi-phase-matched second-harmonic generation (SHG) in the visible range. Detailed analyses of the phase-matching properties for these processes were made by using the published Sellmeier and thermo-optic dispersion formulas. 相似文献
38.
S. K. Godunov V. T. Zhukov O. B. Feodoritova 《Computational Mathematics and Mathematical Physics》2006,46(6):971-982
An algorithm is constructed for calculating invariant subspaces of symmetric hyperbolic systems arising in electromagnetic, acoustic, and elasticity problems. Discrete approximations are calculated for subspaces that correspond to minimal eigenvalues and smooth eigenfunctions. Difficulties related to the presence of an infinite-dimensional kernel in the differential operator are successfully handled. The efficiency of the algorithm is demonstrated using acoustics equations. 相似文献
39.
It is long known that the Fokker-Planck equation with prescribed constant coefficients of diffusion and linear friction describes
the ensemble average of the stochastic evolutions in velocity space of a Brownian test particle immersed in a heat bath of
fixed temperature. Apparently, it is not so well known that the same partial differential equation, but now with constant
coefficients which are functionals of the solution itself rather than being prescribed, describes the kinetic evolution (in
the N→∞ limit) of an
isolated
N-particle system with certain stochastic interactions. Here we discuss in detail this recently discovered interpretation.
An erratum to this article can be found at 相似文献
40.