全文获取类型
收费全文 | 22389篇 |
免费 | 1123篇 |
国内免费 | 1323篇 |
专业分类
化学 | 9236篇 |
晶体学 | 182篇 |
力学 | 122篇 |
综合类 | 39篇 |
数学 | 13320篇 |
物理学 | 1936篇 |
出版年
2023年 | 134篇 |
2022年 | 177篇 |
2021年 | 185篇 |
2020年 | 292篇 |
2019年 | 572篇 |
2018年 | 489篇 |
2017年 | 393篇 |
2016年 | 354篇 |
2015年 | 516篇 |
2014年 | 750篇 |
2013年 | 1971篇 |
2012年 | 931篇 |
2011年 | 1452篇 |
2010年 | 1238篇 |
2009年 | 1332篇 |
2008年 | 1650篇 |
2007年 | 1579篇 |
2006年 | 1236篇 |
2005年 | 955篇 |
2004年 | 847篇 |
2003年 | 726篇 |
2002年 | 851篇 |
2001年 | 571篇 |
2000年 | 566篇 |
1999年 | 560篇 |
1998年 | 490篇 |
1997年 | 414篇 |
1996年 | 387篇 |
1995年 | 419篇 |
1994年 | 388篇 |
1993年 | 295篇 |
1992年 | 259篇 |
1991年 | 169篇 |
1990年 | 169篇 |
1989年 | 148篇 |
1988年 | 114篇 |
1987年 | 104篇 |
1986年 | 110篇 |
1985年 | 136篇 |
1984年 | 128篇 |
1983年 | 76篇 |
1982年 | 100篇 |
1981年 | 100篇 |
1980年 | 101篇 |
1979年 | 77篇 |
1978年 | 85篇 |
1977年 | 53篇 |
1976年 | 48篇 |
1975年 | 27篇 |
1974年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
31.
32.
Ludmila C. Fidale Sarah Köhler Martin H.G. Prechtl Thomas Heinze Omar A. El Seoud 《Cellulose (London, England)》2006,13(5):581-592
The concentrations of water, W, and electrolytes present in solutions of LiCl in N,N-dimethylacetamide, LiCl/DMAc, and of tetrabutylammonium fluoride. x-hydrate in DMSO, TBAF.xW/DMSO can be accurately and expediently determined by three independent methods, UV–vis, FTIR and EMF measurement. The first relies on the use of solvatochromic probes whose spectra are sensitive to solution composition. It is applicable to W/LiCl/DMAc solutions but not to TBAF.xW/DMSO, because the charge-transfer complex bands of the probes are suppressed by strong interactions with the latter electrolyte. Integration of νOH band of water may be employed in order to determine [W], hence [electrolyte] by weight difference. EMF measurement uses ion-selective electrodes in order to determine [electrolyte], hence [W] by weight difference. Results of the latter method were in excellent agreement with those of FTIR. The reason for the failure of Karl Fischer titration is addressed, and the relevance of the results obtained to functionalization of cellulose under homogenous solution conditions is briefly commented on. 相似文献
33.
Summary In this paper a Gauss-Jordan algorithm with column interchanges is presented and analysed. We show that, in contrast with Gaussian elimination, the Gauss-Jordan algorithm has essentially differing properties when using column interchanges instead of row interchanges for improving the numerical stability. For solutions obtained by Gauss-Jordan with column interchanges, a more satisfactory bound for the residual norm can be given. The analysis gives theoretical evidence that the algorithm yields numerical solutions as good as those obtained by Gaussian elimination and that, in most practical situations, the residuals are equally small. This is confirmed by numerical experiments. Moreover, timing experiments on a Cyber 205 vector computer show that the algorithm presented has good vectorisation properties. 相似文献
34.
Using Brown's construction (J. Algebra 15 (1970) 103) of an exact 6-term sequence for a fibration of groupoids we show how an exact 9-term sequence can be associated to a fibration of bigroupoids. Applications to topology and algebra are given. 相似文献
35.
Kazem Ghanbari 《Positivity》2006,10(4):721-729
We denote the spectrum of an square matrix A by σ(A), and that of the matrix obtained by deleting the first i rows and columns of A by σi(A). It is known that a symmetric pentadiagonal oscillatory (SPO) matrix may be constructed from σ, σ1 and σ2. The pairs σ, σ1 and σ1, σ2 must interlace; the construction is not unique; and the conditions on the data which ensure that A is oscillatory are extremely complicated. Given one SPO matrix A, the paper shows that operations may be applied to A to construct a family of such matrices with σ and σ1 in common. Moreover, given one totally positive (TP) matrix A, we construct a family of TP matrices with σ, σ1 and σ2 in common. 相似文献
36.
The graph-theoretic operation of rooted subtree prune and regraft is increasingly being used as a tool for understanding and modelling reticulation events in evolutionary biology. In this paper, we show that computing the rooted subtree prune and regraft distance between two rooted binary phylogenetic trees on the same label set is NP-hard. This resolves a longstanding open problem. Furthermore, we show that this distance is fixed parameter tractable when parameterised by the distance between the two trees.Received March 16, 2004 相似文献
37.
We extend the matrix version of Cochran's statistical theorem to outer inverses of a matrix. As applications, we investigate the Wishartness and independence of matrix quadratic forms for Kronecker product covariance structures. 相似文献
38.
A graph G is N2‐locally connected if for every vertex ν in G, the edges not incident with ν but having at least one end adjacent to ν in G induce a connected graph. In 1990, Ryjá?ek conjectured that every 3‐connected N2‐locally connected claw‐free graph is Hamiltonian. This conjecture is proved in this note. © 2004 Wiley Periodicals, Inc. J Graph Theory 48: 142–146, 2005 相似文献
39.
In this paper we show that for a given 3-manifold and a given Heegaard splitting there are finitely many preferred decomposing systems of 3g−3 disjoint essential disks. These are characterized by a combinatorial criterion which is a slight strengthening of Casson-Gordon's rectangle condition. This is in contrast to fact that in general there can exist infinitely many such systems of disks which satisfy just the Casson-Gordon rectangle condition. 相似文献
40.