全文获取类型
收费全文 | 4510篇 |
免费 | 154篇 |
国内免费 | 380篇 |
专业分类
系统科学 | 271篇 |
丛书文集 | 193篇 |
教育与普及 | 18篇 |
理论与方法论 | 7篇 |
现状及发展 | 47篇 |
综合类 | 4499篇 |
自然研究 | 9篇 |
出版年
2024年 | 3篇 |
2023年 | 23篇 |
2022年 | 31篇 |
2021年 | 41篇 |
2020年 | 36篇 |
2019年 | 56篇 |
2018年 | 47篇 |
2017年 | 48篇 |
2016年 | 74篇 |
2015年 | 126篇 |
2014年 | 187篇 |
2013年 | 151篇 |
2012年 | 229篇 |
2011年 | 234篇 |
2010年 | 188篇 |
2009年 | 256篇 |
2008年 | 262篇 |
2007年 | 343篇 |
2006年 | 288篇 |
2005年 | 288篇 |
2004年 | 259篇 |
2003年 | 236篇 |
2002年 | 213篇 |
2001年 | 190篇 |
2000年 | 197篇 |
1999年 | 160篇 |
1998年 | 130篇 |
1997年 | 103篇 |
1996年 | 101篇 |
1995年 | 82篇 |
1994年 | 81篇 |
1993年 | 68篇 |
1992年 | 75篇 |
1991年 | 65篇 |
1990年 | 54篇 |
1989年 | 60篇 |
1988年 | 35篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1955年 | 2篇 |
排序方式: 共有5044条查询结果,搜索用时 46 毫秒
301.
对n相Barker序列进行了分析,将n相Barker序列划分为若干等价类。等价类元素可通过某种变换相互转化,因此,将n相Barker序列设计简化为等价类的设计。用计算机搜索的方法,得出具有某种长度的所有n相Barker序列等价类。采用{-1,0, 1}作为序列的元素集合,也是取得Barker序列的一种手段。通过与2相Barker序列比较,采用类似2相Barker序列结构,得到的奇数长度的三值Barker序列。虽然该序列具有较长的序列长度,但序列的相关增益较小。通过计算机搜索可以得到任意长度、具有最大相关增益的三值Barker序列。 相似文献
302.
303.
本文主要应用文献[4]的方法,给出了序列|V(n)|的若干新奇的恒等式及一些数论性质。 相似文献
304.
Analysis of components of conserved “backbone sequences”among genomes of Shigella spp. strains 总被引:1,自引:0,他引:1
LIUHong PENGJunping YANGJian SUNLilian CHENShuxia JinQi 《科学通报(英文版)》2004,49(2):152-160
Difference in the genomic compositions of prokaryotes is the basis of the diversity in their biological characters. However, besides their flora-or strain-specific genes,those floras with closer relationship in the evolution also have conserved “backbone sequences”, which reveal the marks of their origin and evolution, and these “backbone sequences”are just the basis of their elementary living abilities and common biological properties. Shigella is very closely related to E. coli in the origin and evolution, and may turn out to belong to the same genus. In this study, a microarray containing E. coli K-12 whole genome and Sf301 specific ORFs is used to investigate the genomic components of four Shigella strains. The results indicate that 16%-22% K-12 ORFs sequences are not detected in the genome of Shigella strains while the genome of Shigella contains at least 2800 conserved ORFs, which compose the common “backbone sequences”.Advanced analysis indicated that the “backbone sequences”are the essential components in maintaining the normal physiological activities of intestinal bacteria. Furthermore,only 20% Sf301-specific ORFs exist in other strains simultaneously, which demonstrate the great genome heterogeneity and the genetic diversity among the strains. 相似文献
305.
基于PVS的UML类图和序列图的一致性检验 总被引:1,自引:0,他引:1
针对UML类图和序列图的一致性问题,在充分考虑了类继承关系、关联关系、类方法的可见性以及类方法的前、后置条件等因素对一致性影响的基础上,给出了判定类图和序列图一致性的必要条件和PVS元理论,提出了一种基于定理证明器PVS的一致性检验方法.在检验UML模型一致性时,把一致性检验问题转化为逻辑定理证明问题.实践表明,该方法对于提高UML模型的可信度,减少系统实现阶段的错误起到了一定作用. 相似文献
306.
Snake venom thrombin-like enzymes: from reptilase to now 总被引:12,自引:0,他引:12
Castro HC Zingali RB Albuquerque MG Pujol-Luz M Rodrigues CR 《Cellular and molecular life sciences : CMLS》2004,61(7-8):843-856
The snake venom thrombin-like enzymes (SVTLEs) comprise a number of serine proteases functionally and structurally related to thrombin. Until recently, only nine complete sequences of this subgroup of the serine protease family were known. Over the past 5 years, the primary structure of several SVTLEs has been characterized, and now this family includes several members. Of particular interest is their possible use in pathologies such as thrombosis. The aim of the present review is to summarize the state of the art concerning the evolutionary, structural and biological features of the SVTLEs.Received 16 August 2003; received after revision 26 September 2003; accepted 1 October 2003 相似文献
307.
Rice LB 《Cellular and molecular life sciences : CMLS》2002,59(12):2023-2032
Among the more important problems in modern hospitals is the prevalence of bacterial pathogens expressing resistance to multiple
antimicrobial agents. The frequency of multiresistance suggests mechanisms by which bacterial species can concentrate and
efficiently exchange a variety of resistance determinants. Mechanisms by which this occurs include insertion of transposons
within transposons, coalescence through the activity of insertion sequences and the employment of integrons. In some instances,
more than one of these mechanisms is involved in creating large multiresistance genetic elements. The association of the elements
with transferable elements or transposons may promote rapid dissemination among clinical strains, and create further opportunities
for inclusion of additional resistance determinants. 相似文献
308.
周明中 《安庆师范学院学报(自然科学版)》2001,7(1):68-70
依 A rrow提出的选举问题的四条公理为准则 ,利用联合尺度条件 ,阐明目前流行的筛选平均分排序法 (即去掉一个最高分和一个最低分后 ,用余下的各分数的算术平均分作为待评人的最后得分 )是一种看似公正 ,实为错误的排序方法 相似文献
309.
Wang Li-na Fei Ru-chun Liu Zhu . State Key Laboratory of Software Engineering Wuhan University Wuhan Hubei China . Department of Information Engneering Benxi College of Metallurgy Benxi Liaoning China . School of Information Science Engineering Northeastern University Shenyang Liaoning China 《武汉大学学报:自然科学英文版》2003,8(4):1065-1069
0 IntroductionAblockcipherwithoutdataextensionorexpressionisaper mutation .Thesecurityofablockcipherschemeiscorrela tivewiththe permutations.Quicktrickle permutation[1 ] canmakethedatadisorderedwhenactingatthedataandmakeallprobabledistancebetweentwoelementsappear,soithasgoodcryptographicpropertiesandcanbeusedintheblockcipherwhichrequireshighsecurity.Aprivatekeyblockcipherisapermutationinn dimensionvectorspaceoverF2 atthecontrolofakey[2 ,3] .Moreover,manyblockciphersusepermutationstoexecuteso… 相似文献
310.
The deep-sea clams Calyptogena nautilei and C. tsubasa, which live in the cold-seep area at a depth of 3570 m in the Nankai Trough, Japan, have abundant hemoglobins (Hbs) in erythrocytes, similar to other Calyptogena species. We determined the cDNA-derived amino acid sequences of Hbs from two Calyptogena species. C. tsubasa was found to contain two dimeric Hbs, Hb I consisting of 145 amino acid residues and Hb II with 137 residues, similar to known Hbs from C. soyoae and C. kaikoi. Sequence identity was over 90% among the orthologous chains of Calyptogena Hbs. On the other hand, surprisingly, C. nautilei contained two monomeric Hbs, Hb III containing 141 residues and Hb IV with 134 residues. In addition, Hbs III and IV showed only 33–42% sequence identity with Hbs I and II from other Calyptogena species. The distal (E7) histidine, one of the functionally important residues of the heme protein, is replaced by glutamine in all Hb chains of Calyptogena species. A phylogenetic analysis indicated that C. nautilei Hb III is closer to Hb I from other Calyptogena species. We suppose that a Hb gene was duplicated at least three times in an immediate ancestor of Calyptogena and, presumably depending on physiological conditions different Hb sets are being expressed: dimeric Hbs I and II in C. soyoae, C. kaikoi and C. tsubasa, and monomeric Hbs III and IV in C. nautilei.
Received 13 May 2003; received after revision 5 June 2003; accepted 12 June 2003 相似文献