全文获取类型
收费全文 | 47765篇 |
免费 | 4986篇 |
国内免费 | 3890篇 |
专业分类
化学 | 25172篇 |
晶体学 | 393篇 |
力学 | 5016篇 |
综合类 | 934篇 |
数学 | 9440篇 |
物理学 | 15686篇 |
出版年
2023年 | 369篇 |
2022年 | 678篇 |
2021年 | 1758篇 |
2020年 | 1219篇 |
2019年 | 1186篇 |
2018年 | 967篇 |
2017年 | 1166篇 |
2016年 | 1422篇 |
2015年 | 1348篇 |
2014年 | 1829篇 |
2013年 | 3107篇 |
2012年 | 2075篇 |
2011年 | 2460篇 |
2010年 | 2145篇 |
2009年 | 2631篇 |
2008年 | 2837篇 |
2007年 | 2944篇 |
2006年 | 2502篇 |
2005年 | 1814篇 |
2004年 | 1656篇 |
2003年 | 1679篇 |
2002年 | 4042篇 |
2001年 | 1751篇 |
2000年 | 1260篇 |
1999年 | 1041篇 |
1998年 | 978篇 |
1997年 | 759篇 |
1996年 | 791篇 |
1995年 | 681篇 |
1994年 | 654篇 |
1993年 | 625篇 |
1992年 | 576篇 |
1991年 | 412篇 |
1990年 | 347篇 |
1989年 | 268篇 |
1988年 | 295篇 |
1987年 | 249篇 |
1986年 | 248篇 |
1985年 | 351篇 |
1984年 | 263篇 |
1983年 | 152篇 |
1982年 | 318篇 |
1981年 | 481篇 |
1980年 | 435篇 |
1979年 | 475篇 |
1978年 | 377篇 |
1977年 | 289篇 |
1976年 | 245篇 |
1974年 | 76篇 |
1973年 | 150篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
41.
主成分分析在地区科技竞争力评测中的应用 总被引:16,自引:0,他引:16
近年来对于科技竞争力的研究在国内方兴未艾,其中对于科技竞争力的评测是众多学者研究的重点和热点,也是各级决策者最为关心、最为重要的课题之一。本文根据科技竞争力概念和内涵来确定评测指标体系的构成要素,建立了评测指标体系,并利用主成分分析方法对采集来的数据进行分析,得到最终的评测结果。 相似文献
42.
In this paper we give a realization of some symmetric space G/K as a closed submanifold P of G. We also give several equivalent representations of the submanifold P. Some properties of the set gK∩P are also discussed, where gK is a coset space in G. 相似文献
43.
The excitation of eigen surface waves by tubular electron beams in cylindrical discharge devices is studied. The influence of the wave‐field azimuthal structure on the excitation efficiency and nonlinear stage of the plasmabeam instability is investigated both numerically and analytically. Analytical expressions for the saturation amplitude and excitation efficiency of the wave under study are derived. They are found to agree well with results obtained by numerical modelling of the plasma‐beam interaction presented in this paper. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
44.
We consider a resource management problem in which the management objective is to minimize fluctuations in resource stocks. Stabilizing management policies constitute the designing of memoryless state feedback control strategies for a discrete time resource model which contains unknown but bounded fluctuations. We also show that the problem of maximizing sustainable yield in an uncertain fishery can be considered as the problem of stabilizing of the stock level.The paper corresponds to an invited talk at the 14th Symposium on Operations Research, Ulm, September 6–8, 1989.The support by Yrjö Jahnsson Foundation is gratefully acknowledged.This work was supported by NSF and AFOSR under grant ECS 8602524. 相似文献
45.
Summary In 1980 Dahmen-DeVore-Scherer introduced a modulus of continuity which turns out to reflect invariance properties of compound cubature rules effectively. Accordingly, sharp error bounds are derived, the existence of relevant counterexamples being a consequence of a quantitative resonance principle, established previously. 相似文献
46.
Camillo Costantini 《Topology and its Applications》2006,153(7):1056-1078
For X a metrizable space and (Y,ρ) a metric space, with Y pathwise connected, we compute the density of (C(X,(Y,ρ)),σ)—the space of all continuous functions from X to (Y,ρ), endowed with the supremum metric σ. Also, for (X,d) a metric space and (Y,‖⋅‖) a normed space, we compute the density of (UC((X,d),(Y,ρ)),σ) (the space of all uniformly continuous functions from (X,d) to (Y,ρ), where ρ is the metric induced on Y by ‖⋅‖). We also prove that the latter result extends only partially to the case where (Y,ρ) is an arbitrary pathwise connected metric space.To carry such an investigation out, the notions of generalized compact and generalized totally bounded metric space, introduced by the author and A. Barbati in a former paper, turn out to play a crucial rôle. Moreover, we show that the first-mentioned concept provides a precise characterization of those metrizable spaces which attain their extent. 相似文献
47.
Michael Carr 《Topology and its Applications》2006,153(12):2155-2168
Given a graph Γ, we construct a simple, convex polytope, dubbed graph-associahedra, whose face poset is based on the connected subgraphs of Γ. This provides a natural generalization of the Stasheff associahedron and the Bott-Taubes cyclohedron. Moreover, we show that for any simplicial Coxeter system, the minimal blow-ups of its associated Coxeter complex has a tiling by graph-associahedra. The geometric and combinatorial properties of the complex as well as of the polyhedra are given. These spaces are natural generalizations of the Deligne-Knudsen-Mumford compactification of the real moduli space of curves. 相似文献
48.
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
49.
论大学物理实验体系建设与教学改革 总被引:3,自引:1,他引:2
传统的物理实验是按普通物理、电子线路和近代物理等内容进行划分,这种体系突出了学科的系统性,但忽视了学科之间的兼容互补性,各学科相互独立,限制了学生跨学科思维能力和创新能力的培养,建立现代物理实验教学新体系是大学物理实验教学改革的必然趋势.新的物理实验体系应依照训练层次,划分为引导性实验、基本实验、综合性实验、设计性实验、研究性实验.并需要注重时代性和先进性,改革和更新大学物理实验教学内容,改革实验教学与管理模式,形成新的运行机制,从而达到学生素质和能力的提高. 相似文献
50.
M. Gorkunov M. Lapine E. Shamonina K.H. Ringhofer 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(3):263-269
Effective magnetic properties of a composite meta-material consisting of periodically arranged circular conductive elements
are studied theoretically. A general expression for the effective bulk permeability is obtained with mutual effects and lattice
ordering being taken into account. The resonance frequency of the permeability is found to be strongly dependent on the size
and shape of the unit cell. Frequency dispersion of the permeability is studied with special attention paid to the frequency
range, where negative values of the permeability are possible. Corresponding recommendations for optimisation of the meta-materials
with negative permeability are made. The results are confirmed by numerical simulations of the finite structure behaviour
in an external magnetic field.
Received 19 April 2002 Published online 31 July 2002 相似文献