全文获取类型
收费全文 | 84766篇 |
免费 | 9016篇 |
国内免费 | 6690篇 |
专业分类
化学 | 34349篇 |
晶体学 | 1171篇 |
力学 | 14175篇 |
综合类 | 1455篇 |
数学 | 24340篇 |
物理学 | 24982篇 |
出版年
2024年 | 131篇 |
2023年 | 724篇 |
2022年 | 1193篇 |
2021年 | 2621篇 |
2020年 | 2254篇 |
2019年 | 2040篇 |
2018年 | 1886篇 |
2017年 | 2369篇 |
2016年 | 2922篇 |
2015年 | 2441篇 |
2014年 | 3623篇 |
2013年 | 5784篇 |
2012年 | 4361篇 |
2011年 | 4893篇 |
2010年 | 4279篇 |
2009年 | 4969篇 |
2008年 | 4987篇 |
2007年 | 5156篇 |
2006年 | 4501篇 |
2005年 | 3673篇 |
2004年 | 3268篇 |
2003年 | 3189篇 |
2002年 | 5352篇 |
2001年 | 2786篇 |
2000年 | 2285篇 |
1999年 | 1999篇 |
1998年 | 1873篇 |
1997年 | 1557篇 |
1996年 | 1443篇 |
1995年 | 1257篇 |
1994年 | 1171篇 |
1993年 | 1043篇 |
1992年 | 1027篇 |
1991年 | 747篇 |
1990年 | 597篇 |
1989年 | 475篇 |
1988年 | 487篇 |
1987年 | 379篇 |
1986年 | 329篇 |
1985年 | 476篇 |
1984年 | 376篇 |
1983年 | 208篇 |
1982年 | 393篇 |
1981年 | 528篇 |
1980年 | 457篇 |
1979年 | 512篇 |
1978年 | 409篇 |
1977年 | 323篇 |
1976年 | 253篇 |
1973年 | 156篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
81.
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
82.
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
83.
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
84.
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
85.
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
86.
许多实验对用CsI(Tl)闪烁晶体作为探测器来寻找和探测暗物质的可行性进行了研究.本工作利用8MeV单能中子轰击CsI(Tl)晶体探测器来研究Cs核和I核的QuenchingFactor.在数据处理中,运用脉冲形状甄别(PSD)方法来分辨反冲核信号和本底信号.实验结果表明,在7keV到132keV的能区中,Quench ingFactor随着反冲核能量的减少而增加.在探测暗物质的实验中,这一性质对于CsI(Tl)晶体探测器获得较低的能量阈值是很有利的. 相似文献
87.
We obtain a solution of the DGLAP equation for the gluon at low x first by expanding the gluon in a Taylor series and then using the method of characteristics. We test its validity by comparing
it with that of Glück, Reya and Vogt. The convergence criteria of the approximation used are also discussed. We also calculate
εF
2(x,Q)2/ε In Q
2 using its approximate relations with the gluon distribution at low x. The predictions are then compared with the HERA data. 相似文献
88.
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
89.
Q. Sun Q. Wang X.G. Gong V. Kumar Y. Kawazoe 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,18(1):77-81
We report results of the atomic and electronic structures of Al7C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is
found to be the one in which carbon atom occupies an interstitial position in Al7 cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al7C- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al7C- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al7N which has the same number of valence electrons as in Al7C- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al7N)2 to study interaction between magic clusters.
Received 28 July 2001 相似文献
90.