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   981篇
  免费   10篇
  国内免费   34篇
系统科学   20篇
丛书文集   2篇
教育与普及   3篇
理论与方法论   12篇
现状及发展   206篇
研究方法   131篇
综合类   625篇
自然研究   26篇
  2021年   4篇
  2020年   5篇
  2019年   5篇
  2018年   14篇
  2017年   12篇
  2016年   8篇
  2015年   13篇
  2014年   18篇
  2013年   18篇
  2012年   67篇
  2011年   95篇
  2010年   37篇
  2009年   24篇
  2008年   52篇
  2007年   66篇
  2006年   52篇
  2005年   63篇
  2004年   39篇
  2003年   64篇
  2002年   53篇
  2001年   40篇
  2000年   42篇
  1999年   20篇
  1996年   4篇
  1992年   10篇
  1991年   7篇
  1990年   4篇
  1989年   10篇
  1988年   9篇
  1987年   9篇
  1986年   5篇
  1985年   9篇
  1982年   6篇
  1981年   7篇
  1980年   8篇
  1979年   8篇
  1978年   7篇
  1977年   10篇
  1976年   5篇
  1975年   9篇
  1974年   5篇
  1973年   9篇
  1972年   3篇
  1971年   9篇
  1970年   9篇
  1969年   7篇
  1968年   5篇
  1967年   7篇
  1966年   5篇
  1965年   7篇
排序方式: 共有1025条查询结果,搜索用时 0 毫秒
41.
42.
For analysts there is a tradeoff between the accuracy and the timeliness of their forecasts. Prior literature heavily investigates analyst forecast accuracy. Few papers investigate the importance of timeliness. To our best knowledge, there are no empirical papers to date to investigate the dynamic interplay between these key characteristics. We show that if analysts experience a period of high accuracy relative to their peers, they subsequently focus more on the timeliness of their forecasts in the subsequent period and thus issue their forecasts earlier than they did in the prior period. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   
43.
44.
45.
Streptococcus pneumoniae remains an important cause of disease with high mortality and morbidity, especially in children and in the elderly. The widespread use of the polysaccharide conjugate vaccines in some countries has led to a significant decrease in invasive disease caused by vaccine serotypes, but an increase in disease caused by non-vaccine serotypes has impacted on the overall efficacy of these vaccines on pneumococcal disease. The obvious solution to overcome such shortcomings would be the development of new formulations that provide serotype-independent immunity. This review focuses on the most promising approaches, including protein antigens, whole cell pneumococcal vaccines, and recombinant bacteria expressing pneumococcal antigens. The protective capacity of these vaccine candidates against the different stages of pneumococcal infection, including colonization, mucosal disease, and invasive disease in animal models is reviewed. Some of the human trials that have already been performed or that are currently ongoing are presented. Finally, the feasibility and the possible shortcomings of these candidates in relation to an ideal vaccine against pneumococcal infections are discussed.  相似文献   
46.
Understory phytomass production in a western juniper ( Juniperus occidentalis ) woodland was examined relative to tree size in central Oregon in 1983 and 1984. Vegetation was sampled in two zones, the canopy zone (beneath the canopy) and the intercanopy zone (the space between canopies), on two adjacent sites—a lower slope site with shallow soil and an upper slope site with deeper soil. Sampling was stratified into three tree size classes. Individual species production was significantly affected by tree size and location relative to tree canopy. Production of bottlebrush squirreltail, bluebunch wheatgrass, cheatgrass, miscellaneous annual grasses, perennial forbs, and annual forbs increased with increasing tree size. Sandberg bluegrass production was greater in the intercanopy than the canopy zone, while production of bottlebrush squirreltail, bluebunch wheatgrass, miscellaneous annual grasses, and both perennial and annual forbs was greater in the canopy zone. Production of cheatgrass was determined by the interaction of tree size and zone. Phytomass relationships were expressed to a greater degree on the upper slope site, where total production exceeded that of the lower slope site by approximately 50% the second year of the study. Individual trees appear to exert a great influence on associated vegetation as western juniper woodlands progress from the seedling (tree establishment) phase to closed stands of mature trees. Original community dominants appear to be spatially segregated beneath tree canopies and associated with large trees, while formerly less common species, such as cheatgrass, come to dominate the entire site.  相似文献   
47.
R D Short  J C Dacre  C C Lee 《Experientia》1979,35(12):1625-1627
Postmitochondrial supernatants prepared from livers of mice, rats, rabbits, dogs, and monkeys metabolized 2,4-dinitrotulene. The pattern of metabolites was characterized in both sexes of the species examined. In addition, the pattern of metabolites was altered by varying incubation conditions and pretreating male rats with phenobarbital or SKF 525-A.  相似文献   
48.
49.
50.
Zusammenfassung Mit einer Analysenmethode gelingt es, Marijuanarauch von einer Zigarette oder weniger durch Kombination von Anreicherungsverfahren mit hochauflösender Gaschromatographie in Zimmerluft nachzuweisen. Charakteristische Profile von 200 bis 300 Komponenten (darunter Cannabinoide), die sich deutlich von denen von Tabakrauch unterscheiden, wurden mit Hilfe von Glaskapillarsäulen gewonnen.  相似文献   
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