全文获取类型
收费全文 | 16367篇 |
免费 | 2032篇 |
国内免费 | 1349篇 |
专业分类
化学 | 4775篇 |
晶体学 | 79篇 |
力学 | 2344篇 |
综合类 | 387篇 |
数学 | 7185篇 |
物理学 | 4978篇 |
出版年
2024年 | 21篇 |
2023年 | 165篇 |
2022年 | 243篇 |
2021年 | 428篇 |
2020年 | 492篇 |
2019年 | 442篇 |
2018年 | 426篇 |
2017年 | 501篇 |
2016年 | 675篇 |
2015年 | 447篇 |
2014年 | 1059篇 |
2013年 | 1546篇 |
2012年 | 886篇 |
2011年 | 1195篇 |
2010年 | 1001篇 |
2009年 | 1112篇 |
2008年 | 978篇 |
2007年 | 932篇 |
2006年 | 804篇 |
2005年 | 757篇 |
2004年 | 674篇 |
2003年 | 658篇 |
2002年 | 731篇 |
2001年 | 482篇 |
2000年 | 468篇 |
1999年 | 386篇 |
1998年 | 361篇 |
1997年 | 308篇 |
1996年 | 243篇 |
1995年 | 212篇 |
1994年 | 166篇 |
1993年 | 148篇 |
1992年 | 133篇 |
1991年 | 110篇 |
1990年 | 82篇 |
1989年 | 71篇 |
1988年 | 54篇 |
1987年 | 40篇 |
1986年 | 41篇 |
1985年 | 42篇 |
1984年 | 35篇 |
1983年 | 22篇 |
1982年 | 27篇 |
1981年 | 24篇 |
1980年 | 19篇 |
1979年 | 24篇 |
1978年 | 16篇 |
1977年 | 15篇 |
1973年 | 10篇 |
1957年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 338 毫秒
81.
In this paper, three new direct Mutually Orthogonal Latin Squares (MOLS) constructions are presented for 7 MOLS(24), 7 MOLS(75) and 8 MOLS(36); then using recursive methods, several new constructions for 7 and 8 MOLS are obtained. These reduce the largest value for which 7 MOLS are unknown from 780 to 570, and the largest odd value for which 8 MOLS are unknown from 1935 to 419. © 2003 Wiley Periodicals, Inc. 相似文献
82.
Whilst the space volume of muffler in noise control system is often constrained for maintenance in practical engineering work, the maximization on muffler’s performance becomes important and essential. In this paper, a novel approach genetic algorithms (GAs) based on the principles of natural biological evolution will be used to tackle this optimization of muffler design [M. Mitchell, An Introduction to Genetic Algorithms, The MIT Press, Cambridge, MA, 1996]. Here, the shape optimization of multi-segments muffler coupled with the GA searching technique is presented. The techniques of binary genetic algorithms (BGA) together with the commercial MATLAB package [G. Lindfield, J. Penny, Numerical Method Using Matlab, second ed., Prentice Hall, Englewood Cliffs, NJ, 2000] are applied in GA searching. In addition, a numerical case of pure tone elimination with 2-5 segments on muffler is introduced and fully discussed. To achieve the best optimization in GA, several GA parameters are on trial in various values. Results show that the GA operators, including crossover mutation and elitism, are essential in accuracy. Consequently, results verify that the optimal sound transmission loss at the designed frequency of 500 Hz is exactly maximized. The GA optimization on multi-segments muffler proposed in this study surely provides a quick and correct approach. 相似文献
83.
E.V. Jansson 《Applied Acoustics》2004,65(12):1197-1205
Good violins have a broad hill in the 2-3 kHz range of their frequency response. This hill has previously been attributed to the first in-plane resonance of the violin bridge. Experiments prove, however, that the hill is the result of two forces acting in opposite directions at the bridge feet. The experiments reported here show that the hill can be “tuned” by altering the distance between the bridge feet. It can be tuned both in terms of frequency and level but the properties of the violin cannot be neglected. 相似文献
84.
A simple and accurate algorithm (phase scanning method) is proposed for 3D surface contouring and dynamic response determination of a vibrating object. A sinusoidal fringe pattern is projected onto a low-frequency vibrating object by a programmable liquid crystal display projector. The fringe patterns are captured by a high-speed CCD camera with a telecentric gauging lens. Phase values are evaluated point by point using phase scanning method. From the phase values of each point on the object, the contour of the specimen at different instants of vibration can be retrieved. In this paper, a small vibrating coin is used to demonstrate the validity of the method and the experimental results are compared with test results on a stationary coin using four-step phase shifting and fast Fourier transform methods. The technique is especially useful in applications where the vibrating object has a complicated shape. 相似文献
85.
Qinghai Wang 《Czechoslovak Journal of Physics》2004,54(1):143-146
This paper investigates finite-dimensional PT-symmetric Hamiltonians. It is shown here that there are two ways to extend real symmetric Hamiltonians into the complex domain: (i) The usual approach is to generalize such Hamiltonians to include complex Hermitian Hamiltonians. (ii) Alternatively, one can generalize real symmetric Hamiltonians to include complex PT-symmetric Hamiltonians. In the first approach the spectrum remains real, while in the second approach the spectrum remains real if the PT symmetry is not broken. Both generalizations give a consistent theory of quantum mechanics, but if D>2, a D-dimensional Hermitian matrix Hamiltonian has more arbitrary parameters than a D-dimensional PT-symmetric matrix Hamiltonian. 相似文献
86.
标度系统一致性矩阵容量的计算方法 总被引:4,自引:0,他引:4
吕跃进 《数学的实践与认识》2003,33(9):102-107
本文研究正互反矩阵的变换性质 ,提出一种保持正互反矩阵一致性程度的等价分类 ,为计算各种标度系统下的一致矩阵容量提供了计算方法 相似文献
87.
Let Um be an m×m Haar unitary matrix and U[m,n] be its n×n truncation. In this paper the large deviation is proven for the empirical eigenvalue density of U[m,n] as m/n→λ and n→∞. The rate function and the limit distribution are given explicitly. U[m,n] is the random matrix model of quq, where u is a Haar unitary in a finite von Neumann algebra, q is a certain projection and they are free. The limit distribution coincides with the Brown measure of the operator quq. 相似文献
88.
§ 1 IntroductionLetRn×mdenotetherealn×mmatrixspace ,Rn×mr itssubsetwhoseelementshaverankr ,ORn×nthesetofalln×northogonalmatrices,SRn×n(SRn×n≥ ,SRn×n>)thesetofalln×nrealsymmetric (symmetricpositivesemidefinite ,positivedefinite)matrices.ThenotationA>0 (≥ 0 ,<0 ,≤ 0 )m… 相似文献
89.
90.