全文获取类型
收费全文 | 4258篇 |
免费 | 322篇 |
国内免费 | 368篇 |
专业分类
化学 | 217篇 |
晶体学 | 1篇 |
力学 | 173篇 |
综合类 | 43篇 |
数学 | 1707篇 |
物理学 | 447篇 |
综合类 | 2360篇 |
出版年
2024年 | 8篇 |
2023年 | 28篇 |
2022年 | 79篇 |
2021年 | 102篇 |
2020年 | 92篇 |
2019年 | 78篇 |
2018年 | 87篇 |
2017年 | 108篇 |
2016年 | 99篇 |
2015年 | 135篇 |
2014年 | 188篇 |
2013年 | 227篇 |
2012年 | 207篇 |
2011年 | 245篇 |
2010年 | 198篇 |
2009年 | 207篇 |
2008年 | 236篇 |
2007年 | 311篇 |
2006年 | 242篇 |
2005年 | 234篇 |
2004年 | 198篇 |
2003年 | 195篇 |
2002年 | 157篇 |
2001年 | 144篇 |
2000年 | 145篇 |
1999年 | 118篇 |
1998年 | 94篇 |
1997年 | 111篇 |
1996年 | 86篇 |
1995年 | 78篇 |
1994年 | 79篇 |
1993年 | 55篇 |
1992年 | 45篇 |
1991年 | 64篇 |
1990年 | 36篇 |
1989年 | 35篇 |
1988年 | 33篇 |
1987年 | 26篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 15篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1980年 | 11篇 |
1979年 | 16篇 |
1978年 | 12篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1974年 | 6篇 |
排序方式: 共有4948条查询结果,搜索用时 31 毫秒
21.
We consider a Bolza optimal control problem with state constraints. It is well known that under some technical assumptions every strong local minimizer of this problem satisfies first order necessary optimality conditions in the form of a constrained maximum principle. In general, the maximum principle may be abnormal or even degenerate and so does not provide a sufficient information about optimal controls. In the recent literature some sufficient conditions were proposed to guarantee that at least one maximum principle is nondegenerate, cf. [A.V. Arutyanov, S.M. Aseev, Investigation of the degeneracy phenomenon of the maximum principle for optimal control problems with state constraints, SIAM J. Control Optim. 35 (1997) 930–952; F. Rampazzo, R.B. Vinter, A theorem on existence of neighbouring trajectories satisfying a state constraint, with applications to optimal control, IMA 16 (4) (1999) 335–351; F. Rampazzo, R.B. Vinter, Degenerate optimal control problems with state constraints, SIAM J. Control Optim. 39 (4) (2000) 989–1007]. Our aim is to show that actually conditions of a similar nature guarantee normality of every nondegenerate maximum principle. In particular we allow the initial condition to be fixed and the state constraints to be nonsmooth. To prove normality we use J. Yorke type linearization of control systems and show the existence of a solution to a linearized control system satisfying new state constraints defined, in turn, by linearization of the original set of constraints along an extremal trajectory. 相似文献
22.
We propose iteration methods for solving the Dirichlet problem in domains with involved geometry. Such problems arise in relation to the problem of optimizing quantum dot and antidot infrared detectors. We estimate the deviation of an approximate solution from the exact solution. 相似文献
23.
This work is concerned with the maximum principle for optimal control problem governed by magnetohydrodynamic equations, which describe the motion of a viscous incompressible conducting fluid in a magnetic field and consist of a subtle coupling of the Navier-Stokes equation of viscous incompressible fluid flow and the Maxwell equation of electromagnetic field. An integral type state constraint is considered. 相似文献
24.
In this work, the NP-hard maximum clique problem on graphs is considered. Starting from basic greedy heuristics, modifications and improvements are proposed and combined in a two-phase heuristic procedure. In the first phase an improved greedy procedure is applied starting from each node of the graph; on the basis of the results of this phase a reduced subset of nodes is selected and an adaptive greedy algorithm is repeatedly started to build cliques around such nodes. In each restart the selection of nodes is biased by the maximal clique generated in the previous execution. Computational results are reported on the DIMACS benchmarks suite. Remarkably, the two-phase procedure successfully solves the difficult Brockington-Culberson instances, and is generally competitive with state-of-the-art much more complex heuristics. 相似文献
25.
Weaving is a matrix construction developed in 1990 for the purpose of obtaining new weighing matrices. Hadamard matrices obtained by weaving have the same orders as those obtained using the Kronecker product, but weaving affords greater control over the internal structure of matrices constructed, leading to many new Hadamard equivalence classes among these known orders. It is known that different classes of Hadamard matrices may have different maximum excess. We explain why those classes with smaller excess may be of interest, apply the method of weaving to explore this question, and obtain constructions for new Hadamard matrices with maximum excess in their respective classes. With this method, we are also able to construct Hadamard matrices of near‐maximal excess with ease, in orders too large for other by‐hand constructions to be of much value. We obtain new lower bounds for the maximum excess among Hadamard matrices in some orders by constructing candidates for the largest excess. For example, we construct a Hadamard matrix with excess 1408 in order 128, larger than all previously known values. We obtain classes of Hadamard matrices of order 96 with maximum excess 912 and 920, which demonstrates that the maximum excess for classes of that order may assume at least three different values. Since the excess of a woven Hadamard matrix is determined by the row sums of the matrices used to weave it, we also investigate the properties of row sums of Hadamard matrices and give lists of them in small orders. © 2004 Wiley Periodicals, Inc. J Combin Designs 12: 233–255, 2004. 相似文献
26.
LetB
1
be a ball of radiusr
1
inS
n
(ℍn), and letB
0
be a smaller ball of radiusr
0
such thatB
0
⊂B
1
. ForS
n
we considerr
1
π. Let u be a solution of the problem- δm = 1 in Ω :=B
1
/B
0
vanishing on the boundary. It is shown that the associated functionalJ (Ω) is minimal if and only if the balls are concentric. It is also shown that the first Dirichlet eigenvalue of the Laplacian
on Ω is maximal if and only if the balls are concentric. 相似文献
27.
Guang-hua DONG~ Yan-pei LIU Department of Mathematics School of Science Beijing Jiaotong University Beijing China 《中国科学A辑(英文版)》2007,50(11):1563-1570
In this paper,we provide a new class of up-embeddable graphs,and obtain a tight lower bound on the maximum genus of a class of 2-connected pseudographs of diameter 2 and of a class of diameter 4 multi-graphs.This extends a result of Skoviera. 相似文献
28.
P. Q. Khanh 《Journal of Optimization Theory and Applications》1995,87(2):359-378
We prove the Kuhn-Tucker sufficient optimality condition, the Wolfe duality, and a modified Mond-Weir duality for vector optimization problems involving various types of invex-convexlike functions. The class of such functins contains many known generalized convex functions. As applications, we demonstrate that, under invex-convexlikeness assumptions, the Pontryagin maximum principle is a sufficient optimality condition for cooperative differential games. The Wolfe duality is established for these games.The author is indebted to the referees and Professor W. Stadler for valuable remarks and comments, which have been used to revise considerably the paper. 相似文献
29.
30.
Detailed pH-dependent steady state and picosecond time-resolved tryptophan fluorescence studies on thiocyanate and azide complexes
of horseradish peroxidase have been carried out. The fluorescence decay of the single tryptophan in these species was fitted
to a discrete three exponential model. Maximum entropy method analysis also gave three distinct regions of lifetime distributions.
The fast subnanosecond lifetime component was found to have > 97% amplitude contribution while other two longer lifetime components
have small contributions. Small contributions from the nanosecond lifetime components possibly arise from apoprotein impurity
or some small amount of disordered heme conformer of the protein. pH dependence of the fast picosecond lifetime components
was found to show a systematic behavior which has been interpreted in the light of obligatory conformation change associated
with activation of the enzyme at low pH. 相似文献