全文获取类型
收费全文 | 247177篇 |
免费 | 3852篇 |
国内免费 | 2097篇 |
专业分类
化学 | 133134篇 |
晶体学 | 3040篇 |
力学 | 10269篇 |
综合类 | 15篇 |
数学 | 29423篇 |
物理学 | 77245篇 |
出版年
2021年 | 2478篇 |
2020年 | 2561篇 |
2019年 | 2792篇 |
2018年 | 3567篇 |
2017年 | 3440篇 |
2016年 | 5420篇 |
2015年 | 3631篇 |
2014年 | 5053篇 |
2013年 | 12187篇 |
2012年 | 10470篇 |
2011年 | 12285篇 |
2010年 | 8097篇 |
2009年 | 8041篇 |
2008年 | 10752篇 |
2007年 | 10850篇 |
2006年 | 10105篇 |
2005年 | 9210篇 |
2004年 | 8741篇 |
2003年 | 7347篇 |
2002年 | 7113篇 |
2001年 | 7354篇 |
2000年 | 5694篇 |
1999年 | 4410篇 |
1998年 | 3553篇 |
1997年 | 3417篇 |
1996年 | 3064篇 |
1995年 | 2927篇 |
1994年 | 2747篇 |
1993年 | 2672篇 |
1992年 | 2976篇 |
1991年 | 2843篇 |
1990年 | 2755篇 |
1989年 | 2519篇 |
1988年 | 2573篇 |
1987年 | 2569篇 |
1986年 | 2399篇 |
1985年 | 3210篇 |
1984年 | 3424篇 |
1983年 | 2776篇 |
1982年 | 2945篇 |
1981年 | 2887篇 |
1980年 | 2728篇 |
1979年 | 2821篇 |
1978年 | 2904篇 |
1977年 | 2910篇 |
1976年 | 2960篇 |
1975年 | 2751篇 |
1974年 | 2715篇 |
1973年 | 2847篇 |
1972年 | 1795篇 |
排序方式: 共有10000条查询结果,搜索用时 578 毫秒
991.
Manufacturing cell formation using similarity coefficients and a parallel genetic TSP algorithm: Formulation and comparison 总被引:2,自引:0,他引:2
J. BalakrishnanP. D. Jog 《Mathematical and Computer Modelling》1995,21(12):61-73
Many algorithms have been proposed to form manufacturing cells from component routings. However, many of these do not have the capability of solving large problems. We propose a procedure using similarity coefficients and a parallel genetic implementation of a TSP algorithm that is capable of solving large problems of up to 1000 parts and 1000 machines. In addition, we also compare our procedure with many existing procedures using nine well-known problems from the literature.
The results show that the proposed procedure compares well with the existing procedures and should be useful to practitioners and researchers. 相似文献
992.
This note outlines an algorithm for solving the complex ‘matrix Procrustes problem’. This is a least‐squares approximation over the cone of positive semi‐definite Hermitian matrices, which has a number of applications in the areas of Optimization, Signal Processing and Control. The work generalizes the method of Allwright (SIAM J. Control Optim. 1988; 26 (3):537–556), who obtained a numerical solution to the real‐valued version of the problem. It is shown that, subject to an appropriate rank assumption, the complex problem can be formulated in a real setting using a matrix‐dilation technique, for which the method of Allwright is applicable. However, this transformation results in an over‐parametrization of the problem and, therefore, convergence to the optimal solution is slow. Here, an alternative algorithm is developed for solving the complex problem, which exploits fully the special structure of the dilated matrix. The advantages of the modified algorithm are demonstrated via a numerical example. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
993.
994.
D. Guyomard C. Pagnoux J. J. Zah Letho A. Verbaere Y. Piffard 《Journal of solid state chemistry》1991,90(2)
The single phase compound Na3SbO(PO4)2 was prepared by a solid state reaction. It crystallizes in the orthorhombic system, space group P212121 with a = 6.964(1) Å, b = 9.284(2) Å, c = 12.425(2) Å, Z = 4. The structure was determined from 968 reflections collected on a Nonius CAD4 automatic diffractometer with MoKα radiation. The final R index and weighted Rw index are 0.031 and 0.039, respectively. The structure is built up from strings of corner-sharing SbO6 octahedra to which phosphate groups are linked by two of their vertices. These chains, running parallel to the a-axis, are separated from each other by sodium atoms. 相似文献
995.
996.
997.
998.
We analyze the critical length for design purposes of six-dimensional spaces invariant under translations and reflections
containing the functions 1, cos t and sin t. These spaces also contain the first degree polynomials as well as trigonometric and/or hyperbolic functions. We identify
the spaces whose critical length for design purposes is greater than 2π and find its maximum 4π. By a change of variables,
two biparametric families of spaces arise. We call shape preservation region to the set of admissible parameters in order
that the space has shape preserving representations for curves. We describe the shape preserving regions for both families.
To our friend Mariano Gasca on occasion of his 60th birthday
Research partially supported by the Spanish Research Grant MTM2006-03388, by Gobierno de Aragón and Fondo Social Europeo. 相似文献
999.
D. Boilley A. Marchix B. Jurado K. -H. Schmidt 《The European Physical Journal A - Hadrons and Nuclei》2007,33(1):47-52
We propose a new formula for the saddle-to-scission time that is more general that the one based on Kramers' approach. Its
validity and applicability is then studied in detail. Such a formula is useful for the evaluation of the fission time of very
heavy nuclei. 相似文献
1000.
The charge distribution of fragments originatingfrom the fission of the 236U compound nucleus is calculated within a stochastic approach based on Langevin equations. The elongation coordinate, the neck-thickness coordinate, and the charge-asymmetry coordinate are chosen as collective variables. The friction parameter of the charge mode is calculated on the basis of two nuclear-viscosity mechanisms, that of one-body and that of two-body dissipation. It is shown that the Langevin approach is applicable to studying isobaric distributions. In addition, the charge distribution in question is studied as a function of the excitation energy of the compound nucleus and as a function of the coefficient of two-body viscosity. 相似文献