全文获取类型
收费全文 | 20018篇 |
免费 | 2761篇 |
国内免费 | 561篇 |
专业分类
化学 | 784篇 |
晶体学 | 14篇 |
力学 | 1036篇 |
综合类 | 330篇 |
数学 | 5571篇 |
物理学 | 3021篇 |
无线电 | 12584篇 |
出版年
2024年 | 74篇 |
2023年 | 261篇 |
2022年 | 461篇 |
2021年 | 538篇 |
2020年 | 539篇 |
2019年 | 456篇 |
2018年 | 472篇 |
2017年 | 683篇 |
2016年 | 893篇 |
2015年 | 912篇 |
2014年 | 1467篇 |
2013年 | 1376篇 |
2012年 | 1419篇 |
2011年 | 1535篇 |
2010年 | 1068篇 |
2009年 | 1219篇 |
2008年 | 1345篇 |
2007年 | 1400篇 |
2006年 | 1187篇 |
2005年 | 985篇 |
2004年 | 839篇 |
2003年 | 726篇 |
2002年 | 622篇 |
2001年 | 482篇 |
2000年 | 441篇 |
1999年 | 319篇 |
1998年 | 303篇 |
1997年 | 249篇 |
1996年 | 220篇 |
1995年 | 155篇 |
1994年 | 108篇 |
1993年 | 115篇 |
1992年 | 102篇 |
1991年 | 46篇 |
1990年 | 56篇 |
1989年 | 36篇 |
1988年 | 42篇 |
1987年 | 23篇 |
1986年 | 30篇 |
1985年 | 35篇 |
1984年 | 32篇 |
1983年 | 13篇 |
1982年 | 18篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1959年 | 5篇 |
1957年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
应用遗传算法设计和开发了针对爱立信GSM900系统的频率规划软件。在设计中,采用了交叉点存储、变异回退以及分割分步降维的方法,从而提高遗传算法的执行速度。 相似文献
22.
An Improved Immune Genetic Algorithm for Solving the Optimization Problems of Computer Communication Networks 总被引:4,自引:0,他引:4
1 IntroductionIndesigningacomputercommunicationnet work ,thenetworkaveragedelayisanimportantpa rameterinthenetworkperformance .Inthispaper,weonlyconsiderM /M/1networks,whichmeansthatthemessageprocessingtimeisaprobabilisticdensityfunctionwithnegativepower,thegroupar rivalandsendingisofPoissiondistributionwithasinglequeue .Supposethatthenetworktopologicalstructureandtheestimatesoftheexternaltrafficrequirementsaregiven ,howtoselecttheoptimalroutestobeusedbythecommunicatingnodesinthenetworksoast… 相似文献
23.
24.
25.
In this paper, we propose a new reduced-complexity decoding algorithm of Low-Density Parity-Check (LDPC) codes, called Belief-Propagation-Approximated (BPA) algorithm, which utilizes the idea of normalization and translates approximately the intricate nonlinear operation in the check nodes of the original BP algorithm to only one operation of looking up the table. The normalization factors can be obtained by simulation, or theoretically. Simulation results demonstrate that BPA algorithm exhibits fairly satisfactory bit error performance on the Additive White Gaussian Noise (AWGN) channel. 相似文献
26.
Resilient optical networks are predominately designed to protect against single failures of fiber links. But in larger networks, operators also see dual failures. As the capacity was planned for single failures, disconnections can occur by dual failures even if enough topological connectivity is provided. In our approach the design of the network minimizes the average loss caused by dual failures, while single failures are still fully survived. High dual failure restorability is the primary aim, capacity is optimized in a second step. For WDM networks with full wavelength conversion, we formulate mixed integer linear programming models for dedicated path protection, shared (backup) path protection, and path rerouting with and without stub-release. For larger problem instances in path rerouting, we propose two heuristics. Computational results indicate that the connectivity is of much more importance for high restorability values than the overall protection capacity. Shared protection has similar restorability levels as dedicated protection while the capacity is comparable to rerouting. Rerouting surpasses the protection mechanisms in restorability and comes close to 100% dual failure survivability. Compared to single failure planning, both shared path protection and rerouting need significantly more capacity in dual failure planning. 相似文献
27.
28.
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
29.
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
30.
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