全文获取类型
收费全文 | 23022篇 |
免费 | 3023篇 |
国内免费 | 714篇 |
专业分类
化学 | 1616篇 |
晶体学 | 28篇 |
力学 | 1097篇 |
综合类 | 368篇 |
数学 | 7035篇 |
物理学 | 3350篇 |
无线电 | 13265篇 |
出版年
2024年 | 65篇 |
2023年 | 230篇 |
2022年 | 461篇 |
2021年 | 581篇 |
2020年 | 576篇 |
2019年 | 492篇 |
2018年 | 507篇 |
2017年 | 746篇 |
2016年 | 966篇 |
2015年 | 990篇 |
2014年 | 1593篇 |
2013年 | 1585篇 |
2012年 | 1647篇 |
2011年 | 1726篇 |
2010年 | 1187篇 |
2009年 | 1384篇 |
2008年 | 1597篇 |
2007年 | 1618篇 |
2006年 | 1337篇 |
2005年 | 1142篇 |
2004年 | 983篇 |
2003年 | 852篇 |
2002年 | 734篇 |
2001年 | 599篇 |
2000年 | 550篇 |
1999年 | 422篇 |
1998年 | 385篇 |
1997年 | 323篇 |
1996年 | 302篇 |
1995年 | 203篇 |
1994年 | 151篇 |
1993年 | 162篇 |
1992年 | 133篇 |
1991年 | 67篇 |
1990年 | 67篇 |
1989年 | 53篇 |
1988年 | 54篇 |
1987年 | 40篇 |
1986年 | 38篇 |
1985年 | 45篇 |
1984年 | 42篇 |
1983年 | 15篇 |
1982年 | 23篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1979年 | 14篇 |
1978年 | 4篇 |
1977年 | 17篇 |
1976年 | 7篇 |
1959年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 7 毫秒
31.
改进型遗传算法在加载天线设计中的运用 总被引:5,自引:2,他引:3
遗传算法是一种全新的优化搜索方法,可以用来解决各种复杂的实际问题。针对简单型遗传算法的一些不足之处,介绍了一种改进型的遗传算法,并阐述了其在短波宽带集总加载天线优化设计中的运用。 相似文献
32.
In this paper we consider a wavelet algorithm for the piecewise constant collocation method applied to the boundary element solution of a first kind integral equation arising in acoustic scattering. The conventional stiffness matrix is transformed into the corresponding matrix with respect to wavelet bases, and it is approximated by a compressed matrix. Finally, the stiffness matrix is multiplied by diagonal preconditioners such that the resulting matrix of the system of linear equations is well conditioned and sparse. Using this matrix, the boundary integral equation can be solved effectively. 相似文献
33.
杨晓光 《应用数学学报(英文版)》1997,13(4):337-341
1.IntroductionIntillspaperweanalyzetheconvergenceonmultiplicativeiterativealgorithmsfortheIninimizationofadiffcrentiablefunctiondefinedonthepositiveorthantofR".ThealgorithmissllggestedbyEggermolltl'],andisrelatedtotheEM[2](Expextation--Maximization)algoritllnlforPositronemissiontonlography[']andimagereconstructi..14].Wecollsidertheproblenl"linf(x)s.t.x20.Themultiplicativeiterativealgorithmshavethel'orlniforj=l,2,',n,withAhdeterminedthroughalinesearch.Whilelusem[5]establishedanelegantconv… 相似文献
34.
Summary This paper is concerned with the problem of developing numerical integration algorithms for differential equations that, when
viewed as equations in some Euclidean space, naturally evolve on some embedded submanifold. It is desired to construct algorithms
whose iterates also evolve on the same manifold. These algorithms can therefore be viewed as integrating ordinary differential
equations on manifolds. The basic method “decouples” the computation of flows on the submanifold from the numerical integration
process. It is shown that two classes of single-step and multistep algorithms can be posed and analyzed theoretically, using
the concept of “freezing” the coefficients of differential operators obtained from the defining vector field. Explicit third-order
algorithms are derived, with additional equations augmenting those of their classical counterparts, obtained from “obstructions”
defined by nonvanishing Lie brackets. 相似文献
35.
Jinchao Xu 《应用数学学报(英文版)》2002,18(2):185-200
Abstract Some new local and parallel finite element algorithms are proposed and analyzed in this paper foreigenvalue problems.With these algorithms, the solution of an eigenvalue problem on a fine grid is reduced tothe solution of an eigenvalue problem on a relatively coarse grid together with solutions of some linear algebraicsystems on fine grid by using some local and parallel procedure.A theoretical tool for analyzing these algorithmsis some local error estimate that is also obtained in this paper for finite element approximations of eigenvectorson general shape-regular grids. 相似文献
36.
The performance of sequential decoding of long constraint length convolutional codes is evaluated for Rayleigh fading channels. Sequential decoding is not practical below a certain theoretical signal-to-noise ratio, and these theoretical limits are calculated for a number of modulation methods and code rates. As an example, with BPSK modulation, soft decisions and code rate 1/2, the theoretical signal-to-noise ratio per information bit is 5.7 dB. Above this limit the bit error rate can be made arbitrarily small by increasing the constraint length at no significant complexity cost. Furthermore, it is shown that with carefully chosen quantization steps, 8 level uniform quantization gives a negligible loss also for sequential decoding on a Rayleigh fading channel. Simulation results using 8 level quantization correspond well with the theoretical performance bounds. Also, the performance on a correlated channel with finite interleaving has been obtained. With an interleaver depth of 50×50 and a normalized doppler frequency equal to 0.01 we are only 0.5 dB away from the performance with perfect interleaving. Finally, bit error rate results show this scheme to compete well with Turbo codes. 相似文献
37.
M. M. Kostreva 《Journal of Optimization Theory and Applications》1989,62(1):63-76
Murty's algorithm for the linear complementarity problem is generalized to solve the optimality conditions for linear and convex quadratic programming problems with both equality and inequality constraints. An implementation is suggested which provides both efficiency and tight error control. Numerical experiments as well as field tests in various applications show favorable results.The author thanks K. G. Murty for his encouragement and helpful comments. 相似文献
38.
In this paper, the authors studied certain properties of the estimate of Liang and Krishnaiah (1985, J. Multivariate Anal. 16, 162–172) for multivariate binary density. An alternative shrinkage estimate is also obtained. The above results are generalized to general orthonormal systems. 相似文献
39.
We examine two criteria for balance of a gain graph, one based on binary cycles and one on circles. The graphs for which each criterion is valid depend on the set of allowed gain groups. The binary cycle test is invalid, except for forests, if any possible gain group has an element of odd order. Assuming all groups are allowed, or all abelian groups, or merely the cyclic group of order 3, we characterize, both constructively and by forbidden minors, the graphs for which the circle test is valid. It turns out that these three classes of groups have the same set of forbidden minors. The exact reason for the importance of the ternary cyclic group is not clear. © 2005 Wiley Periodicals, Inc. J Graph Theory 相似文献
40.
R. J. Vanderbei 《Mathematical Programming》1989,43(1-3):31-44
The affine-scaling modification of Karmarkar's algorithm is extended to solve problems with free variables. This extended primal algorithm is used to prove two important results. First the geometrically elegant feasibility algorithm proposed by Chandru and Kochar is the same algorithm as the one obtained by appending a single column of residuals to the constraint matrix. Second the dual algorithm as first described by Adler et al., is the same as the extended primal algorithm applied to the dual. 相似文献