全文获取类型
收费全文 | 30068篇 |
免费 | 118篇 |
国内免费 | 275篇 |
专业分类
系统科学 | 211篇 |
丛书文集 | 533篇 |
教育与普及 | 54篇 |
理论与方法论 | 102篇 |
现状及发展 | 13482篇 |
研究方法 | 1287篇 |
综合类 | 14383篇 |
自然研究 | 409篇 |
出版年
2013年 | 316篇 |
2012年 | 457篇 |
2011年 | 856篇 |
2010年 | 212篇 |
2009年 | 175篇 |
2008年 | 570篇 |
2007年 | 644篇 |
2006年 | 623篇 |
2005年 | 607篇 |
2004年 | 583篇 |
2003年 | 580篇 |
2002年 | 668篇 |
2001年 | 1070篇 |
2000年 | 947篇 |
1999年 | 687篇 |
1992年 | 602篇 |
1991年 | 426篇 |
1990年 | 489篇 |
1989年 | 502篇 |
1988年 | 467篇 |
1987年 | 550篇 |
1986年 | 474篇 |
1985年 | 597篇 |
1984年 | 484篇 |
1983年 | 365篇 |
1982年 | 340篇 |
1981年 | 367篇 |
1980年 | 464篇 |
1979年 | 889篇 |
1978年 | 758篇 |
1977年 | 740篇 |
1976年 | 612篇 |
1975年 | 632篇 |
1974年 | 848篇 |
1973年 | 757篇 |
1972年 | 780篇 |
1971年 | 841篇 |
1970年 | 1075篇 |
1969年 | 813篇 |
1968年 | 823篇 |
1967年 | 796篇 |
1966年 | 681篇 |
1965年 | 473篇 |
1959年 | 249篇 |
1958年 | 440篇 |
1957年 | 292篇 |
1956年 | 259篇 |
1955年 | 246篇 |
1954年 | 239篇 |
1948年 | 162篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
1.
文本提出了一种区别于传统自主访问控制策略和强制访问控制策略的RBAC网络安全策略,并提出了RBAC的面向对象模型,实现用户身份认证与访问权限的逻辑分离,且易于控制,既能有效的改进系统安全性能,又极大地减少了权限管理的负担。 相似文献
2.
3.
4.
M.Hasegawa 《复旦学报(自然科学版)》2007,(5)
1 Results In the coordination system by using complexation with organic ligand, the ff emission of lanthanide(Ⅲ) (Ln(Ⅲ)) is induced the excitation energy transfer form the organic chromophore under the light-irradiation. However, there are not so much number of reports to discuss the energy relaxation mechanism in such complexes with Ln(Ⅲ). Recently, we succeeded firstly to estimate the rate constant of the energy transfer between the ligand and Ln(Ⅲ) in Pr(Ⅲ)-phenanthroline analogs[1]. Here, we will di... 相似文献
5.
6.
7.
Finding finer functions for partially characterized proteins by protein-protein interaction networks
LI YanHui GUO Zheng MA WenCai YANG Da WANG Dong ZHANG Min ZHU ding ZHONG GuoCai LI YongJin YAO Chen WANG Jing 《科学通报(英文版)》2007,52(24):3363-3370
Based on high-throughput data, numerous algorithms have been designed to find functions of novel proteins. However, the effectiveness of such algorithms is currently limited by some fundamental factors, including (1) the low a-priori probability of novel proteins participating in a detailed function; (2) the huge false data present in high-throughput datasets; (3) the incomplete data coverage of functional classes; (4) the abundant but heterogeneous negative samples for training the algorithms; and (5) the lack of detailed functional knowledge for training algorithms. Here, for partially characterized proteins, we suggest an approach to finding their finer functions based on protein interaction sub-networks or gene expression patterns, defined in function-specific subspaces. The proposed approach can lessen the above-mentioned problems by properly defining the prediction range and functionally filtering the noisy data, and thus can efficiently find proteins’ novel functions. For thousands of yeast and human proteins partially characterized, it is able to reliably find their finer functions (e.g., the translational functions) with more than 90% precision. The predicted finer functions are highly valuable both for guiding the follow-up wet-lab validation and for providing the necessary data for training algorithms to learn other proteins. 相似文献
8.
9.
10.
P pili in uropathogenic E. coli are composite fibres with distinct fibrillar adhesive tips. 总被引:25,自引:0,他引:25
Escherichia coli is a frequent cause of several common bacterial infections in humans and animals, including urinary tract infections, bacteraemia and bacteria-related diarrhoea and is also the main cause of neonatal meningitis. Microbial attachment to surfaces is a key event in colonization and infection and results mainly from a stereochemical fit between microbial adhesins and complementary receptors on host cells. Bacterial adhesins required for extracellular colonization by Gram-negative bacteria are often minor components of heteropolymeric fibres called pili which must be oriented in an accessible manner in these structures to be able to bind to specific receptor architectures. P pili mediate the binding of uropathogenic E. coli to a digalactoside receptor determinant present in the urinary tract epithelium. We report here that the adhesin is a component of distinct fibrillar structures present at the tips of the pili. These virulence-associated tip fibrillae are thin, flexible polymers composed mostly of repeating subunits of PapE that frequently terminate with the alpha-D-galactopyranosyl-(1-4)-beta-D-galactopyranose or Gal alpha (1-4)Gal binding PapG adhesin. 相似文献