全文获取类型
收费全文 | 19930篇 |
免费 | 882篇 |
国内免费 | 175篇 |
专业分类
化学 | 13648篇 |
晶体学 | 125篇 |
力学 | 540篇 |
数学 | 3475篇 |
物理学 | 3199篇 |
出版年
2023年 | 162篇 |
2022年 | 331篇 |
2021年 | 451篇 |
2020年 | 485篇 |
2019年 | 470篇 |
2018年 | 330篇 |
2017年 | 297篇 |
2016年 | 759篇 |
2015年 | 591篇 |
2014年 | 710篇 |
2013年 | 1092篇 |
2012年 | 1397篇 |
2011年 | 1649篇 |
2010年 | 875篇 |
2009年 | 756篇 |
2008年 | 1239篇 |
2007年 | 1177篇 |
2006年 | 1151篇 |
2005年 | 1055篇 |
2004年 | 929篇 |
2003年 | 637篇 |
2002年 | 642篇 |
2001年 | 235篇 |
2000年 | 185篇 |
1999年 | 193篇 |
1998年 | 152篇 |
1997年 | 169篇 |
1996年 | 218篇 |
1995年 | 148篇 |
1994年 | 141篇 |
1993年 | 150篇 |
1992年 | 108篇 |
1991年 | 126篇 |
1990年 | 89篇 |
1989年 | 92篇 |
1988年 | 98篇 |
1987年 | 84篇 |
1986年 | 89篇 |
1985年 | 125篇 |
1984年 | 130篇 |
1983年 | 74篇 |
1982年 | 143篇 |
1981年 | 132篇 |
1980年 | 101篇 |
1979年 | 89篇 |
1978年 | 120篇 |
1977年 | 97篇 |
1976年 | 80篇 |
1975年 | 73篇 |
1973年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Francisco‐Javier Sayas 《Numerical Methods for Partial Differential Equations》2003,19(5):555-570
This article presents and analyzes a simple method for the exterior Laplace equation through the coupling of finite and boundary element methods. The main novelty is the use of a smooth parametric artificial boundary where boundary elements fit without effort together with a straight approximate triangulation in the bounded area, with the coupling done only in nodes. A numerically integrated version of the algorithm is also analyzed. Finally, an isoparametric variant with higher order is proposed. © 2003 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 19: 555–570, 2003 相似文献
62.
The difference in reactivity of the two modifications of InCl in solid state reactions with SnCl2 is discussed. It is explained on the basis of semi‐empirical and ab initio calculations giving the density of states diagrams of InCl and the energies of possible disproportionation reactions. Their general features are discussed on the basis of a simple bonding picture for open‐packed structures involving inert pair elements. A detailed analysis of the DOS distribution allows to pinpoint the observed redox instability to a specific structural feature of α‐InCl. 相似文献
63.
We characterize the sequences of orthogonal polynomials on the unit circle whose derivatives are also orthogonal polynomials on the unit circle. Some relations for the sequences of derivatives of orthogonal polynomials are provided. Finally, we pose some problems about orthogonality-preserving maps and differential equations for orthogonal polynomials on the unit circle. 相似文献
64.
Block-diagonalization of sparse equivariant discretization matrices is studied. Such matrices typically arise when partial
differential equations that evolve in symmetric geometries are discretized via the finite element method or via finite differences.
By considering sparse equivariant matrices as equivariant graphs, we identify a condition for when block-diagonalization via
a sparse variant of a generalized Fourier transform (GFT) becomes particularly simple and fast.
Characterizations for finite element triangulations of a symmetric domain are given, and formulas for assembling the block-diagonalized
matrix directly are presented. It is emphasized that the GFT preserves symmetric (Hermitian) properties of an equivariant
matrix.
By simulating the heat equation at the surface of a sphere discretized by an icosahedral grid, it is demonstrated that the
block-diagonalization is beneficial. The gain is significant for a direct method, and modest for an iterative method.
A comparison with a block-diagonalization approach based upon the continuous formulation is made. It is found that the sparse
GFT method is an appropriate way to discretize the resulting continuous subsystems, since the spectrum and the symmetry are
preserved.
AMS subject classification (2000) 43A30, 65T99, 20B25 相似文献
65.
66.
67.
The anti-Parkinson’s agent SIB-1508Y was prepared in six steps from (S)-nicotine in 20% overall yield. The strategy involves a regioselective formylation at C-5 of a 1,4-dihydronicotine intermediate. 相似文献
68.
Daniel Wasserfallen 《Tetrahedron》2006,62(23):5417-5420
By using drastic conditions for a Diels-Alder cycloaddition reaction, it was possible to synthesize an oligophenylene with an extremely dense packing of the benzene rings. Crystallographic data could be obtained and a projection of the structure on the plane of the central phenyl ring reveals that the molecule retained its theoretical threefold symmetry with only minor deviations. Due to its dense packing of interlocked benzene rings, this oligophenylene could be furthermore used as a suitable precursor for constructing a subunit of ‘cubic graphite’. 相似文献
69.
Ignacio Moreno Pablo Velásquez Daniel Puerto Asticio Vargas 《Optics Communications》2006,260(1):12-18
In this work, we show a very simple birefringent filter useful to modulate a broadband light spectrum. We first show the generation of various oscillating spectra using two identical multiple-order waveplates. In particular a sinusoidal broadband spectrum with a rapid oscillation is obtained. This spectrum is amplitude modulated by adding another waveplate to the system. This waveplate has a small phase shift, and with the proper orientation generates amplitude beats on the broadband spectrum. Finally, we show how this system can be voltage controlled by means of a ferroelectric liquid crystal modulator. We experimentally demonstrate this behavior with the aid of a portable spectrophotometer, and we provide a theoretical explanation on the basis of the Jones matrix formalism. 相似文献
70.
Summary The optimality question for blockp-cyclic SOR iterations discussed in Young and Varga is answered under natural conditions on the spectrum of the block Jacobi matrix. In particular, it is shown that repartitioning a blockp-cyclic matrix into a blockq-cyclic form,q
, results in asymptotically faster SOR convergence for the same amount of work per iteration. As a consequence block 2-cyclic SOR is optimal under these conditions.Research supported in part by the US Air Force under Grant no. AFOSR-88-0285 and the National Science Foundation under grant no. DMS-85-21154 Present address: Boeing Computer Services, P.O. Box 24346, MS 7L-21, Seattle, WA 98124-0346, USA 相似文献