首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   43349篇
  免费   94篇
  国内免费   146篇
系统科学   212篇
丛书文集   826篇
教育与普及   115篇
理论与方法论   231篇
现状及发展   19298篇
研究方法   1681篇
综合类   20699篇
自然研究   527篇
  2013年   300篇
  2012年   576篇
  2011年   1117篇
  2010年   258篇
  2008年   754篇
  2007年   782篇
  2006年   822篇
  2005年   830篇
  2004年   734篇
  2003年   792篇
  2002年   768篇
  2001年   1334篇
  2000年   1217篇
  1999年   805篇
  1994年   373篇
  1992年   786篇
  1991年   619篇
  1990年   681篇
  1989年   673篇
  1988年   642篇
  1987年   685篇
  1986年   682篇
  1985年   857篇
  1984年   670篇
  1983年   570篇
  1982年   523篇
  1981年   499篇
  1980年   603篇
  1979年   1398篇
  1978年   1180篇
  1977年   1114篇
  1976年   835篇
  1975年   909篇
  1974年   1245篇
  1973年   1074篇
  1972年   1072篇
  1971年   1332篇
  1970年   1754篇
  1969年   1330篇
  1968年   1284篇
  1967年   1276篇
  1966年   1126篇
  1965年   820篇
  1959年   483篇
  1958年   729篇
  1957年   565篇
  1956年   496篇
  1955年   433篇
  1954年   470篇
  1948年   298篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
We develop in this paper an efficient way to select the best subset threshold autoregressive model. The proposed method uses a stochastic search idea. Differing from most conventional approaches, our method does not require us to fix the delay or the threshold parameters in advance. By adopting the Markov chain Monte Carlo techniques, we can identify the best subset model from a very large of number of possible models, and at the same time estimate the unknown parameters. A simulation experiment shows that the method is very effective. In its application to the US unemployment rate, the stochastic search method successfully selects lag one as the time delay and five best models from more than 4000 choices. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
92.
P Villa  J Courtin 《Nature》1991,351(6328):613-614
  相似文献   
93.
P Meyrand  J Simmers  M Moulins 《Nature》1991,351(6321):60-63
  相似文献   
94.
Analysis of 78 Huntington's disease (HD) chromosomes with multi-allele markers revealed 26 different haplotypes, suggesting a variety of independent HD mutations. The most frequent haplotype, accounting for about one third of disease chromosomes, suggests that the disease gene is between D4S182 and D4S180. However, the paucity of an expected class of chromosomes that can be related to this major haplotype by assuming single crossovers may reflect the operation of other mechanisms in creating haplotype diversity. Some of these mechanisms sustain alternative scenarios that do not require a multiple mutational origin for HD and/or its positioning between D4S182 and D4S180.  相似文献   
95.
T lymphocytes recognize antigen in the form of peptides that associate with specific alleles of class I or class II major histocompatibility (MHC) molecules. By contrast with the clear MHC allele-specific binding of peptides to purified class II molecules purified solubilized class I molecules either bind relatively poorly or show degenerate specificity. Using photo-affinity labelling, we demonstrate here the specific interaction of peptides with cell-associated MHC class I molecules and show that this involves metabolically active processes.  相似文献   
96.
Several hundred million tons of toxic mercurials are dispersed in the biosphere. Microbes can detoxify organo-mercurials and mercury salts through sequential action of two enzymes, organomercury lyase and mercuric ion reductase (MerA). The latter, a homodimer with homology to the FAD-dependent disulphide oxidoreductases, catalyses the reaction NADPH + Hg(II)----NADP+ + H+ + Hg(0), one of the very rare enzymic reactions with metal substrates. Human glutathione reductase serves as a reference molecule for FAD-dependent disulphide reductases and between its primary structure and that of MerA from Tn501 (Pseudomonas), Tn21 (Shigella), p1258 (Staphylococcus) and Bacillus, 25-30% of the residues have been conserved. All MerAs have a C-terminal extension about 15 residues long but have very varied N termini. Although the enzyme from Streptomyces lividans has no addition, from Pseudomonas aeruginosa Tn501 and Bacillus sp. strain RC607 it has one and two copies respectively of a domain of 80-85 residues, highly homologous to MerP, the periplasmic component of proteins encoded by the mer operon. These domains can be proteolytically cleaved off without changing the catalytic efficiency. We report here the crystal structure of MerA from the Gram-positive bacterium Bacillus sp. strain RC607. Analysis of its complexes with nicotinamide dinucleotide substrates and the inhibitor Cd(II) reveals how limited structural changes enable an enzyme to accept as substrate what used to be a dangerous inhibitor. Knowledge of the mode of mercury ligation is a prerequisite for understanding this unique detoxification mechanism.  相似文献   
97.
J M Nunnari  D L Zimmerman  S C Ogg  P Walter 《Nature》1991,352(6336):638-640
The rough endoplasmic reticulum membranes of mammalian cells contain specific ribosome-binding sites. A purification to apparent homogeneity of a negatively charged protein (ERp180) of relative molecular mass 180,000 (180 K) was reported which was proposed to function as a rough endoplasmic reticulum ribosome receptor. We report here that ribosome-binding site activity quantitatively solubilized from rough endoplasmic reticulum membranes does not cofractionate with ERp180. By contrast, ribosome-binding site activity fractionates as a much smaller, positively charged protein.  相似文献   
98.
BARNOTHY J 《Nature》1948,161(4096):681
  相似文献   
99.
100.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号