全文获取类型
收费全文 | 8854篇 |
免费 | 1078篇 |
国内免费 | 780篇 |
专业分类
化学 | 920篇 |
晶体学 | 21篇 |
力学 | 828篇 |
综合类 | 147篇 |
数学 | 5717篇 |
物理学 | 3079篇 |
出版年
2023年 | 83篇 |
2022年 | 103篇 |
2021年 | 139篇 |
2020年 | 180篇 |
2019年 | 217篇 |
2018年 | 192篇 |
2017年 | 221篇 |
2016年 | 281篇 |
2015年 | 220篇 |
2014年 | 337篇 |
2013年 | 822篇 |
2012年 | 424篇 |
2011年 | 504篇 |
2010年 | 430篇 |
2009年 | 545篇 |
2008年 | 626篇 |
2007年 | 597篇 |
2006年 | 612篇 |
2005年 | 496篇 |
2004年 | 406篇 |
2003年 | 421篇 |
2002年 | 388篇 |
2001年 | 331篇 |
2000年 | 304篇 |
1999年 | 278篇 |
1998年 | 256篇 |
1997年 | 179篇 |
1996年 | 164篇 |
1995年 | 120篇 |
1994年 | 113篇 |
1993年 | 81篇 |
1992年 | 67篇 |
1991年 | 57篇 |
1990年 | 58篇 |
1989年 | 41篇 |
1988年 | 55篇 |
1987年 | 38篇 |
1986年 | 32篇 |
1985年 | 46篇 |
1984年 | 41篇 |
1983年 | 19篇 |
1982年 | 36篇 |
1981年 | 21篇 |
1980年 | 22篇 |
1979年 | 23篇 |
1978年 | 22篇 |
1977年 | 16篇 |
1976年 | 11篇 |
1974年 | 9篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The Burton-Miller boundary integral formulation is solved by a complex variable boundary element-free method (CVBEFM) for the boundary-only meshless analysis of acoustic problems with arbitrary wavenumbers. To regularize both strongly singular and hypersingular integrals and to avoid the computation of the solid angle and its normal derivative, a weakly singular Burton-Miller formulation is derived by considering the normal derivative of the solid angle and adopting the singularity subtraction procedures. To facilitate the implementation of the CVBEFM and the approximation of gradients of the boundary variables, a stabilized complex variable moving least-square approximation is selected in the meshless discretization procedure. The results show the accuracy and efficiency of the present CVBEFM and reveal that the method can produce satisfactory results for all wavenumbers, even for extremely large wavenumbers such as k = 10 000. 相似文献
2.
Thomas Marmin Prof. Yves L. Dory 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(27):6707-6711
Cyclohexane and cyclotri-β-alanyl have been used as scaffolds for the design of new C3-symmetric rings incorporating conjugated alkenes and dienes. All three C3-symmetric lactams share the same triangular shape and their crystal system is trigonal. They all belong to the R3 space group, R3m, R3 and R3c, for the increasingly large 12-, 18- and 24-membered rigid rings, respectively. All lactams stack on top of each other, through H-bonds and van der Waals noncovalent interactions, leading to endless supramolecular cylinders and tubes. The largest member of the family leads to tubes, the central pores of which is wide enough to let water in. A common feature of all the lactams is their very large dipole, of around 9 D, according to DFT calculations. Surprisingly, all the resulting cylinders and tubes pack side by side in the crystals, with all the dipoles pointing to the same direction. As a result, all three crystals are anisotropic and appear to be the first members of a new kind of highly polar crystals. 相似文献
3.
In this paper, we investigate the evolution of joint invariants under invariant geometric flows using the theory of equivariant moving frames and the induced invariant discrete variational complex. For certain arc length preserving planar curve flows invariant under the special Euclidean group , the special linear group , and the semidirect group , we find that the induced evolution of the discrete curvature satisfies the differential‐difference mKdV, KdV, and Burgers' equations, respectively. These three equations are completely integrable, and we show that a recursion operator can be constructed by precomposing the characteristic operator of the curvature by a certain invariant difference operator. Finally, we derive the constraint for the integrability of the discrete curvature evolution to lift to the evolution of the discrete curve itself. 相似文献
4.
5.
We employ a first-principles plane wave method with the
relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter
(HGH) scheme in the frame of DFT to calculate
the equilibrium lattice parameters and the thermodynamic properties of
AlB2 compound with hcp structure. The obtained lattice parameters are in
good agreement with the available experimental data and those calculated by
others. Through the quasi-harmonic Debye model, obtained successfully are
the dependences of the
normalized lattice parameters a/a0 and c/c0 on pressure P, the normalized
primitive cell volume V/V0 on pressure P, the variation of the thermal
expansion α with pressure P and temperature T, as well as the Debye
temperature \ThetaD and the heat capacity CV on pressure P and
temperature T. 相似文献
6.
7.
Zhong-Qing Wang 《Journal of Mathematical Analysis and Applications》2002,274(1):374-403
An orthogonal system of rational functions is discussed. Some inverse inequalities, imbedding inequalities and approximation results are obtained. Two model problems are considered. The stabilities and convergences of proposed rational spectral schemes and rational pseudospectral schemes are proved. The techniques used in this paper are also applicable to other problems on the whole line. Numerical results show the efficiency of this approach. 相似文献
8.
Formalization for problems of multicriteria decision making under uncertainty is constructed in terms of guaranteed and weak
estimates. A relevant definition of the vector maximinimax value is given. Parameterization and approximation of maximum,
minimax, and maximinimax values based on the inverse logical convolution are suggested. An application for multicommodity
networks is considered.
Received: December 13, 2000 / Accepted: August 21, 2001?Published online May 8, 2002 相似文献
9.
10.
This paper is concerned with the implementation and testing of an algorithm for solving constrained least-squares problems. The algorithm is an adaptation to the least-squares case of sequential quadratic programming (SQP) trust-region methods for solving general constrained optimization problems. At each iteration, our local quadratic subproblem includes the use of the Gauss–Newton approximation but also encompasses a structured secant approximation along with tests of when to use this approximation. This method has been tested on a selection of standard problems. The results indicate that, for least-squares problems, the approach taken here is a viable alternative to standard general optimization methods such as the Byrd–Omojokun trust-region method and the Powell damped BFGS line search method. 相似文献