全文获取类型
收费全文 | 24375篇 |
免费 | 1753篇 |
国内免费 | 1252篇 |
专业分类
化学 | 10426篇 |
晶体学 | 123篇 |
力学 | 125篇 |
综合类 | 60篇 |
数学 | 14427篇 |
物理学 | 2219篇 |
出版年
2024年 | 33篇 |
2023年 | 318篇 |
2022年 | 261篇 |
2021年 | 270篇 |
2020年 | 540篇 |
2019年 | 766篇 |
2018年 | 756篇 |
2017年 | 494篇 |
2016年 | 654篇 |
2015年 | 739篇 |
2014年 | 1105篇 |
2013年 | 1931篇 |
2012年 | 1045篇 |
2011年 | 1533篇 |
2010年 | 1290篇 |
2009年 | 1576篇 |
2008年 | 1745篇 |
2007年 | 1728篇 |
2006年 | 1357篇 |
2005年 | 941篇 |
2004年 | 885篇 |
2003年 | 813篇 |
2002年 | 695篇 |
2001年 | 489篇 |
2000年 | 492篇 |
1999年 | 548篇 |
1998年 | 510篇 |
1997年 | 344篇 |
1996年 | 426篇 |
1995年 | 439篇 |
1994年 | 389篇 |
1993年 | 308篇 |
1992年 | 265篇 |
1991年 | 154篇 |
1990年 | 150篇 |
1989年 | 143篇 |
1988年 | 116篇 |
1987年 | 97篇 |
1986年 | 100篇 |
1985年 | 139篇 |
1984年 | 107篇 |
1983年 | 67篇 |
1982年 | 114篇 |
1981年 | 95篇 |
1980年 | 83篇 |
1979年 | 67篇 |
1978年 | 74篇 |
1977年 | 54篇 |
1976年 | 61篇 |
1975年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 323 毫秒
41.
Yevhen Zelenyuk 《Topology and its Applications》2006,153(14):2382-2385
We introduce the notion of a partially selective ultrafilter and prove that (a) if G is an extremally disconnected topological group and p is a converging nonprincipal ultrafilter on G containing a countable discrete subset, then p is partially selective, and (b) the existence of a nonprincipal partially selective ultrafilter on a countable set implies the existence of a P-point in ω∗. Thus it is consistent with ZFC that there is no extremally disconnected topological group containing a countable discrete nonclosed subset. 相似文献
42.
Francis Jordan 《Topology and its Applications》2006,153(14):2386-2412
An abstract scheme using particular types of relations on filters leads to general unifying results on stability under supremum and product of local topological properties. We present applications for Fréchetness, strong Fréchetness, countable tightness and countable fan-tightness, some of which recover or refine classical results, some of which are new. The reader may find other applications as well. 相似文献
43.
Saida Sultanic 《Integral Equations and Operator Theory》2006,55(4):573-595
The central question of this paper is the one of finding the right analogue of the Commutant Lifting Theorem for the Bergman
space La2. We also analyze the analogous problem for weighted Bergman spaces La,α2, − 1 < α < ∞. 相似文献
44.
45.
用全实加关联方法计算了类锂V20+离子1s23d-1s2nf的跃迁能和偶极振子强度.依据量子亏损理论, 确定了1s2nf系列的量子数亏损,用这些作为能量的缓变函数的量子亏损,实现对该Rydberg系列任意高激发态(n≥10)的能量的可靠预言.将这些分立态振子强度与量子亏损理论相结合,得到在电离域附近束缚态间的偶极跃迁振子强度以及束缚态-连续态跃迁的振子强度密度,从而将V20+离子的这一重要光谱特性的理论预言外推到整个能域. 相似文献
46.
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
47.
The paper provides significant simplifications and extensions of results obtained by Gorsich, Genton, and Strang (J. Multivariate Anal. 80 (2002) 138) on the structure of spatial design matrices. These are the matrices implicitly defined by quadratic forms that arise naturally in modelling intrinsically stationary and isotropic spatial processes. We give concise structural formulae for these matrices, and simple generating functions for them. The generating functions provide formulae for the cumulants of the quadratic forms of interest when the process is Gaussian, second-order stationary and isotropic. We use these to study the statistical properties of the associated quadratic forms, in particular those of the classical variogram estimator, under several assumptions about the actual variogram. 相似文献
48.
Parallel to Cox's [JRSS B34 (1972) 187-230] proportional hazards model, generalized logistic models have been discussed by Anderson [Bull. Int. Statist. Inst. 48 (1979) 35-53] and others. The essential assumption is that the two densities ratio has a known parametric form. A nice property of this model is that it naturally relates to the logistic regression model for categorical data. In astronomic, demographic, epidemiological, and other studies the variable of interest is often truncated by an associated variable. This paper studies generalized logistic models for the two-sample truncated data problem, where the two lifetime densities ratio is assumed to have the form exp{α+φ(x;β)}. Here φ is a known function of x and β, and the baseline density is unspecified. We develop a semiparametric maximum likelihood method for the case where the two samples have a common truncation distribution. It is shown that inferences for β do not depend the nonparametric components. We also derive an iterative algorithm to maximize the semiparametric likelihood for the general case where different truncation distributions are allowed. We further discuss how to check goodness of fit of the generalized logistic model. The developed methods are illustrated and evaluated using both simulated and real data. 相似文献
49.
F. Balibrea 《Topology and its Applications》2006,153(12):2092-2095
Recently, Forti, Paganoni and Smítal constructed an example of a triangular map of the unite square, F(x,y)=(f(x),g(x,y)), possessing periodic orbits of all periods and such that no infinite ω-limit set of F contains a periodic point. In this note we show that the above quoted map F has a homoclinic orbit. As a consequence, we answer in the negative the problem presented by A.N. Sharkovsky in the eighties whether, for a triangular map of the square, existence of a homoclinic orbit implies the existence of an infinite ω-limit set containing a periodic point. It is well known that, for a continuous map of the interval, the answer is positive. 相似文献
50.
Chuanming Zong 《Monatshefte für Mathematik》2002,134(3):247-255
In this article we study the simultaneous packing and covering constants of two-dimensional centrally symmetric convex domains.
Besides an identity result between translative case and lattice case and a general upper bound, exact values for some special
domains are determined. Similar to Mahler and Reinhardt’s result about packing densities, we show that the simultaneous packing
and covering constant of an octagon is larger than that of a circle.
(Received 17 January 2001; in revised form 13 July 2001) 相似文献