全文获取类型
收费全文 | 3615篇 |
免费 | 139篇 |
国内免费 | 26篇 |
专业分类
化学 | 2537篇 |
晶体学 | 19篇 |
力学 | 54篇 |
数学 | 567篇 |
物理学 | 603篇 |
出版年
2023年 | 27篇 |
2022年 | 65篇 |
2021年 | 100篇 |
2020年 | 87篇 |
2019年 | 107篇 |
2018年 | 121篇 |
2017年 | 81篇 |
2016年 | 138篇 |
2015年 | 127篇 |
2014年 | 146篇 |
2013年 | 201篇 |
2012年 | 245篇 |
2011年 | 262篇 |
2010年 | 179篇 |
2009年 | 113篇 |
2008年 | 250篇 |
2007年 | 244篇 |
2006年 | 220篇 |
2005年 | 191篇 |
2004年 | 152篇 |
2003年 | 122篇 |
2002年 | 101篇 |
2001年 | 49篇 |
2000年 | 35篇 |
1999年 | 38篇 |
1998年 | 36篇 |
1997年 | 24篇 |
1996年 | 33篇 |
1995年 | 23篇 |
1994年 | 29篇 |
1993年 | 17篇 |
1992年 | 11篇 |
1991年 | 14篇 |
1989年 | 8篇 |
1988年 | 13篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 11篇 |
1983年 | 13篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 12篇 |
1979年 | 9篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 15篇 |
1975年 | 12篇 |
1973年 | 7篇 |
1954年 | 5篇 |
排序方式: 共有3780条查询结果,搜索用时 15 毫秒
951.
We prove a selection theorem which unifies the nonconvex-valued and the zero-dimensional selection theorems. Our proof is based on a new method which unifies the methods of outside and inside approximations of a given lower semicontinuous multivalued mapping. As an application, we obtain a unified selection theorem ('modulo" a countable subset of the domain) and a theorem on density of selections. 相似文献
952.
In this paper we examine dynamical modes resulting from diffusion-like interaction of two model biochemical cells. Kinetics in each of the cells is given by the ICC model of calcium ions in the cytosol. Constraints for one of the cells are set so that it is excitable. One of the constraints in the other cell - a fraction of activated cell surface receptors-is varied so that the dynamics in the cell is either excitable or oscillatory or a stable focus. The cells are interacting via mass transfer and dynamics of the coupled system are studied as two parameters are varied-the fraction of activated receptors and the coupling strength. We find that (i) the excitator-excitator interaction does not lead to oscillatory patterns, (ii) the oscillator-excitator interaction leads to alternating phase-locked periodic and quasiperiodic regimes, well known from oscillator-oscillator interactions; torus breaking bifurcation generates chaos when the coupling strength is in an intermediate range, (iii) the focus-excitator interaction generates compound oscillations arranged as period adding sequences alternating with chaotic windows; the transition to chaos is accompanied by period doublings and folding of branches of periodic orbits and is associated with a Shilnikov homoclinic orbit. The nature of spontaneous self-organized oscillations in the focus-excitator range is discussed. (c) 1999 American Institute of Physics. 相似文献
953.
We study the initial value problem for the two-dimensional nonlinear nonlocal Schrödinger equations i ut + u = N(v), (t, x, y) R3, u(0, x, y) = u0(x, y), (x, y) R2 (A), where the Laplacian = 2
x + 2
y, the solution u is a complex valued function, the nonlinear term N = N1 + N2 consists of the local nonlinear part N1(v) which is cubic with respect to the vector v=(u,ux,uy,\overline{u},\overline{u}_{x},\overline{u}_{y}) in the neighborhood of the origin, and the nonlocal nonlinear part N2(v) =(v, – 1
x Kx(v)) + (v, – 1
y Ky(v)), where (, ) denotes the inner product,
and the vectors Kx (C4(C6; C))6 and Ky (C4(C6; C))6 are quadratic with respect to the vector v in the neighborhood of the origin. We assume that the components K(2)
x = K(4)
x 0, K(3)
y = K(6)
y 0. In particular, Equation (A) includes two physical examples appearing in fluid dynamics. The elliptic–hyperbolic Davey–Stewartson system can be reduced to Equation (A) with
, and all the rest components of the vectors Kx and Ky are equal to zero. The elliptic–hyperbolic Ishimori system is involved in Equation (A), when
, and
. Our purpose in this paper is to prove the local existence in time of small solutions to the Cauchy problem (A) in the usual Sobolev space, and the global-in-time existence of small solutions to the Cauchy problem (A) in the weighted Sobolev space under some conditions on the complex conjugate structure of the nonlinear terms, namely if N(ei v) = ei N(v) for all R. 相似文献
954.
Gyula?KárolyiEmail author Pavel?ValtrEmail author 《Discrete and Computational Geometry》2003,30(2):277-286
In this paper we give a lower bound for
the Erd\H os–Szekeres number in higher dimensions.
Namely, in two different ways
we construct, for every $n>d\ge 2$,
a configuration of $n$ points
in general position in $\R^d$ containing
at most $c_d(\log n)^{d-1}$ points in convex position.
(Points in $\R^d$ are in convex position if
none of them lies in the convex hull of the others.) 相似文献
955.
Pavel Drábek Gabriela Holubová Aleš Matas Petr Nečesal 《Applications of Mathematics》2003,48(6):497-514
In this paper we present several nonlinear models of suspension bridges; most of them have been introduced by Lazer and McKenna. We discuss some results which were obtained by the authors and other mathematicians for the boundary value problems and initial boundary value problems. Our intention is to point out the character of these results and to show which mathematical methods were used to prove them instead of giving precise proofs and statements. 相似文献
956.
We show that for every odd integer p 1 there is an absolute positive
constantcp, so that the maximum cardinality of a set of vectors in Rn
such that the lp distance between
any pair is precisely 1, is at most cp
n log n.
We prove some upper bounds for other lp norms as well. 相似文献
957.
In this paper we consider the analogue of the Courant nodal domain theorem for the nonlinear eigenvalue problem for the p-Laplacian. In particular we prove that if uλn is an eigenfunction associated with the nth variational eigenvalue, λn, then uλn has at most 2n−2 nodal domains. Also, if uλn has n+k nodal domains, then there is another eigenfunction with at most n−k nodal domains. 相似文献
958.
Pavel Pudlák 《Combinatorica》2002,22(2):321-334
Dedicated to the memory of Paul Erdős
We consider the problem of finding some structure in the zero-nonzero pattern of a low rank matrix. This problem has strong
motivation from theoretical computer science. Firstly, the well-known problem on rigidity of matrices, proposed by Valiant
as a means to prove lower bounds on some algebraic circuits, is of this type. Secondly, several problems in communication
complexity are also of this type. The special case of this problem, where one considers positive semidefinite matrices, is
equivalent to the question of arrangements of vectors in euclidean space so that some condition on orthogonality holds. The
latter question has been considered by several authors in combinatorics [1, 4]. Furthermore, we can think of this problem
as a kind of Ramsey problem, where we study the tradeoff between the rank of the adjacency matrix and, say, the size of a
largest complete subgraph. In this paper we show that for an real matrix with nonzero elements on the main diagonal, if the rank is o(n), the graph of the nonzero elements of the matrix contains certain cycles. We get more information for positive semidefinite
matrices.
Received September 9, 1999
RID="*"
ID="*" Partially supported by grant A1019901 of the Academy of Sciences of the Czech Republic and by a cooperative research
grant INT-9600919/ME-103 from the NSF (USA) and the MŠMT (Czech Republic). 相似文献
959.
Resonance Raman (RR) spectroscopy, combined with Kerr gated fluorescence rejection in the time domain, has recently elucidated lignin structure with unique sensitivity and selectivity. This promises structural studies of fluorescent natural macromolecules, such as lignin, which were previously not possible. Such studies rely on an improved understanding of the RR spectral behavior of lignin, which is today scarcely understood. We explain for the first time this behavior by a semi-empirical theory, and observe its pertinent features for lignin in vascular plants. We have used well-defined oxidative treatments as means of probing lignin structural elements, and show that RR sensitivity and selectivity depend crucially on excitation wavelength. Through the theory we relate these results to basic structural aspects of lignin. Spectra obtained by blue light laser excitation (400 nm) are dominated by low redox potential syringyl lignin groups, whereas lower photon energy (500 nm) decreases the selectivity markedly. RR bands depend on molecular structure but also on molecular environment. Thus charge transfer donor-acceptor interactions within lignin reduce the intensity of bands associated with electron rich moieties. New possibilities for basic and selective structural information on fluorescent natural materials, such as lignin, have thus appeared. 相似文献
960.
Efficient copper(II) acetate-catalyzed N-arylation of carbamate-protected hydrazones was achieved under mild reaction conditions with organobismuthanes. 相似文献