全文获取类型
收费全文 | 33133篇 |
免费 | 840篇 |
国内免费 | 750篇 |
专业分类
化学 | 6819篇 |
晶体学 | 959篇 |
力学 | 491篇 |
综合类 | 48篇 |
数学 | 21383篇 |
物理学 | 5023篇 |
出版年
2023年 | 79篇 |
2022年 | 255篇 |
2021年 | 197篇 |
2020年 | 232篇 |
2019年 | 637篇 |
2018年 | 681篇 |
2017年 | 447篇 |
2016年 | 487篇 |
2015年 | 492篇 |
2014年 | 911篇 |
2013年 | 2392篇 |
2012年 | 902篇 |
2011年 | 2304篇 |
2010年 | 1997篇 |
2009年 | 2119篇 |
2008年 | 2358篇 |
2007年 | 2324篇 |
2006年 | 1729篇 |
2005年 | 1401篇 |
2004年 | 1249篇 |
2003年 | 1015篇 |
2002年 | 955篇 |
2001年 | 640篇 |
2000年 | 646篇 |
1999年 | 679篇 |
1998年 | 658篇 |
1997年 | 536篇 |
1996年 | 629篇 |
1995年 | 636篇 |
1994年 | 590篇 |
1993年 | 478篇 |
1992年 | 497篇 |
1991年 | 347篇 |
1990年 | 312篇 |
1989年 | 330篇 |
1988年 | 217篇 |
1987年 | 231篇 |
1986年 | 187篇 |
1985年 | 276篇 |
1984年 | 244篇 |
1983年 | 132篇 |
1982年 | 208篇 |
1981年 | 233篇 |
1980年 | 164篇 |
1979年 | 113篇 |
1978年 | 165篇 |
1977年 | 128篇 |
1976年 | 121篇 |
1975年 | 53篇 |
1974年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
The high-temperature cubic phase of non-stoichiometric strontium ferrite SrFeOx (2.5≤x≤3.0) has been studied by in situ neutron powder diffraction in air over the temperature range 300-1273 K. The composition of SrFeOx changes within the range 2.56≤x≤2.81 from 1273 to 673 K, respectively.Rietveld refinements of the diffraction patterns show that the high-temperature cubic phase of SrFeOx is consistent with a face-centred Fm3c structure. This structure leads to agreement with previous density measurements. This cell allows the high-temperature structure of SrFeOx to be described in terms of a solid solution of the composition end members. Cubic SrFeOx at high temperature is found to closely obey Vegard's law. The density of cubic SrFeOx is also found to exhibit a linear relationship with composition. 相似文献
52.
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
53.
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
54.
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
55.
Fillmore在[1]中得到一个定理:设A,T是Banach空间X上的线性变换,A有界,若Lat(A) Lat(T)且AT=TA,则T是A的多项式.在本文里,以此作为引理,讨论了Banach空间上可逆线性变换A在什么情况下,A-1可表示为A的多项式.本文最主要的结论是定理3.4:设X是Banach空间,A是X上的有界线性变换,且可逆,则A-1是A的多项式当且仅当A-1是A的局部多项式. 相似文献
56.
Kunyu Guo 《Journal of Functional Analysis》2004,213(2):380-411
This paper mainly concerns defect operators and defect functions of Hardy submodules, Bergman submodules over the unit ball, and Hardy submodules over the polydisk. The defect operator (function) carries key information about operator theory (function theory) and structure of analytic submodules. The problem when a submodule has finite defect is attacked for both Hardy submodules and Bergman submodules. Our interest will be in submodules generated by polynomials. The reason for choosing such submodules is to understand the interaction of operator theory, function theory and algebraic geometry. 相似文献
57.
We show that there are close relations between extremal problems in dual Brunn-Minkowski theory and isotropic-type properties for some Borel measures on the sphere. The methods we use allow us to obtain similar results in the context of Firey-Brunn-Minkowski theory. We also study reverse inequalities for dual mixed volumes which are related with classical positions, such as ?-position or isotropic position. 相似文献
58.
Sauer, Shelah, Vapnik and Chervonenkis proved that if a set system on n vertices contains many sets, then the set system has full trace on a large set. Although the restriction on the size of the
groundset cannot be lifted, Frankl and Pach found a trace structure that is guaranteed to occur in uniform set systems even
if we do not bound the size of the groundset. In this note we shall give three sequences of structures such that every set
system consisting of sufficiently many sets contains at least one of these structures with many sets. 相似文献
59.
Using the theory of noncommutative symmetric functions, we introduce the higher order peak algebras (Sym(N))N≥1, a sequence of graded Hopf algebras which contain the descent algebra and the usual peak algebra as initial cases (N=1 and N=2). We compute their Hilbert series, introduce and study several combinatorial bases, and establish various algebraic identities
related to the multisection of formal power series with noncommutative coefficients.
Received November 19, 2004 相似文献
60.
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