全文获取类型
收费全文 | 116311篇 |
免费 | 9911篇 |
国内免费 | 14273篇 |
专业分类
化学 | 60832篇 |
晶体学 | 2193篇 |
力学 | 9648篇 |
综合类 | 1751篇 |
数学 | 33873篇 |
物理学 | 32198篇 |
出版年
2024年 | 155篇 |
2023年 | 914篇 |
2022年 | 1587篇 |
2021年 | 2149篇 |
2020年 | 2589篇 |
2019年 | 2759篇 |
2018年 | 2375篇 |
2017年 | 2952篇 |
2016年 | 3675篇 |
2015年 | 3301篇 |
2014年 | 4478篇 |
2013年 | 7938篇 |
2012年 | 6461篇 |
2011年 | 5848篇 |
2010年 | 5089篇 |
2009年 | 6800篇 |
2008年 | 7366篇 |
2007年 | 7633篇 |
2006年 | 7085篇 |
2005年 | 6244篇 |
2004年 | 5929篇 |
2003年 | 5569篇 |
2002年 | 5364篇 |
2001年 | 4093篇 |
2000年 | 4046篇 |
1999年 | 3505篇 |
1998年 | 3223篇 |
1997年 | 2667篇 |
1996年 | 2461篇 |
1995年 | 2196篇 |
1994年 | 2029篇 |
1993年 | 1635篇 |
1992年 | 1631篇 |
1991年 | 1180篇 |
1990年 | 972篇 |
1989年 | 816篇 |
1988年 | 757篇 |
1987年 | 589篇 |
1986年 | 463篇 |
1985年 | 551篇 |
1984年 | 501篇 |
1983年 | 260篇 |
1982年 | 406篇 |
1981年 | 452篇 |
1980年 | 342篇 |
1979年 | 337篇 |
1978年 | 246篇 |
1977年 | 225篇 |
1976年 | 169篇 |
1973年 | 120篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Carsten Franz 《Proceedings of the American Mathematical Society》2006,134(8):2437-2446
A method to approximate the eigenvalues of linear operators depending on an unknown distribution is introduced and applied to weighted sums of squared normally distributed random variables. This area of application includes the approximation of the asymptotic null distribution of certain degenerated U- and V-statistics.
992.
Two matrix approximation problems are considered: approximation of a rectangular complex matrix by subunitary matrices with
respect to unitarily invariant norms and a minimal rank approximation with respect to the spectral norm. A characterization
of a subunitary approximant of a square matrix with respect to the Schatten norms, given by Maher, is extended to the case
of rectangular matrices and arbitrary unitarily invariant norms. Iterative methods, based on the family of Gander methods
and on Higham’s scaled method for polar decomposition of a matrix, are proposed for computing subunitary and minimal rank
approximants. Properties of Gander methods are investigated in details.
AMS subject classification (2000) 65F30, 15A18 相似文献
993.
本文研究并揭示了L_(2~a)(2~(2~a-1))型正交表行(列)间的递推规律,提出了一种泛函梯度数值计算的新方法——正交试验法,该方法在计算速度和精度上优于直接梯度法;在通用性及节省内存方面优于伴随算子法。 相似文献
994.
本文介绍了兰州重离子加速器(HIRFL)的注入器(SFC)所用PIG离子源的研制和改进工作,使用新研制的PIG源,已在注人器SFC上获得了5μA的O_(16)~(5+)及10μA的C_(16)~(4+)的离子束。 相似文献
995.
Shoaib Ahmad 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,18(3):309-318
The mechanisms and processes of the formation of the regenerative soot in a graphite hollow cathode discharge that produces
and emits carbon clusters are presented. Mass spectrometry with a specially designed E×B velocity filter analyzes the entire range of the charged clusters from C
1 to ∼C
4300. The state of the carbon vapour within the source is evaluated by using the characteristic line emissions from the carbonaceous
discharge whose formative mechanisms depend upon the kinetic and potential sputtering of the sooted cathode. The carbonaceous
discharge generates atomic and ionic C and its clusters C
m (m≥ 2), noble gas metastable atoms and ions, energetic electrons and photons in the cavity of the graphite hollow cathode. The
parameters of soot formation and its recycling depend critically on the discharge parameters, the geometry of the hollow cathode
and 3D profile of the cusp magnetic field contours.
Received 2nd July 2001 and Received in final form 10 September 2001 相似文献
996.
Thomas Andreae 《Journal of Graph Theory》2002,39(4):222-229
For a graph A and a positive integer n, let nA denote the union of n disjoint copies of A; similarly, the union of ?0 disjoint copies of A is referred to as ?0A. It is shown that there exist (connected) graphs A and G such that nA is a minor of G for all n??, but ?0A is not a minor of G. This supplements previous examples showing that analogous statements are true if, instead of minors, isomorphic embeddings or topological minors are considered. The construction of A and G is based on the fact that there exist (infinite) graphs G1, G2,… such that Gi is not a minor of Gj for all i ≠ j. In contrast to previous examples concerning isomorphic embeddings and topological minors, the graphs A and G presented here are not locally finite. The following conjecture is suggested: for each locally finite connected graph A and each graph G, if nA is a minor of G for all n ? ?, then ?0A is a minor of G, too. If true, this would be a far‐reaching generalization of a classical result of R. Halin on families of disjoint one‐way infinite paths in graphs. © 2002 Wiley Periodicals, Inc. J Graph Theory 39: 222–229, 2002; DOI 10.1002/jgt.10016 相似文献
997.
998.
999.
The asymptotic null distribution of the likelihood ratio test for two cases of ordered hypotheses in a particular genetic model is considered. A simple iterative process is proposed in order to get the restricted estimates. It is shown that both tests have asymptotically a chi-bar squared distribution and the same size. A simulation study is also conducted in order to compare the usual unrestricted test with the corresponding one of ordered hypotheses. Finally, the results are extended to some special cases. 相似文献
1000.