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   5291篇
  免费   398篇
  国内免费   6篇
  2023年   25篇
  2022年   27篇
  2021年   121篇
  2020年   82篇
  2019年   77篇
  2018年   127篇
  2017年   100篇
  2016年   165篇
  2015年   259篇
  2014年   252篇
  2013年   365篇
  2012年   414篇
  2011年   434篇
  2010年   250篇
  2009年   212篇
  2008年   278篇
  2007年   325篇
  2006年   309篇
  2005年   262篇
  2004年   273篇
  2003年   238篇
  2002年   237篇
  2001年   59篇
  2000年   48篇
  1999年   46篇
  1998年   57篇
  1997年   29篇
  1996年   31篇
  1995年   34篇
  1994年   28篇
  1993年   30篇
  1992年   35篇
  1991年   36篇
  1990年   30篇
  1989年   16篇
  1988年   20篇
  1987年   33篇
  1986年   24篇
  1985年   30篇
  1984年   26篇
  1983年   16篇
  1982年   21篇
  1981年   21篇
  1980年   17篇
  1979年   19篇
  1976年   15篇
  1974年   12篇
  1973年   17篇
  1971年   12篇
  1970年   14篇
排序方式: 共有5695条查询结果,搜索用时 15 毫秒
1.
The advent of publishing anonymized call detail records opens the door for temporal and spatial human dynamics studies. Such studies, besides being useful for creating universal models for mobility patterns, could be also used for creating new socio-economic proxy indicators that will not rely only on the local or state institutions. In this paper, from the frequency of calls at different times of the day, in different small regional units (sub-prefectures) in Côte d''Ivoire, we infer users'' home and work sub-prefectures. This division of users enables us to analyze different mobility and calling patterns for the different regions. We then compare how those patterns correlate to the data from other sources, such as: news for particular events in the given period, census data, economic activity, poverty index, power plants and energy grid data. Our results show high correlation in many of the cases revealing the diversity of socio-economic insights that can be inferred using only mobile phone call data. The methods and the results may be particularly relevant to policy-makers engaged in poverty reduction initiatives as they can provide an affordable tool in the context of resource-constrained developing economies, such as Côte d''Ivoire''s.  相似文献   
2.
3.
4.
5.
6.
7.
While magnetoreception in birds has been studied intensively, the literature on magnetoreception in bony fish, and particularly in non-migratory fish, is quite scarce. We examined alignment of common carps (Cyprinus carpio) at traditional Christmas sale in the Czech Republic. The sample comprised measurements of the directional bearings in 14,537 individual fish, distributed among 80 large circular plastic tubs, at 25 localities in the Czech Republic, during 817 sampling sessions, on seven subsequent days in December 2011. We found that carps displayed a statistically highly significant spontaneous preference to align their bodies along the North-South axis. In the absence of any other common orientation cues which could explain this directional preference, we attribute the alignment of the fish to the geomagnetic field lines. It is apparent that the display of magnetic alignment is a simple experimental paradigm of great heuristic potential.  相似文献   
8.
Calmodulin coupled to Sepharose has provided a rapid and sensitive means of isolating a cyclic nucleotide phosphodiesterase activity which is stimulated by the calmodulin-Ca2+ complex, from rat parotid gland. Initial experiments established that phosphodiesterase activity sensitive to calmodulin and Ca2+ could not be demonstrated in crude extracts of rat parotid gland or after partial purification of rat parotid phosphodiesterase over DEAE-cellulose. However, it was possible to readily demonstrate the presence of a cyclic nucleotide phosphodiesterase activity regulated by calmodulin if the extracts were first purified by batch ion-exchange chromatography over DEAE-cellulose followed by affinity chromatography with calmodulin coupled to Sepharose. The batch ion-exchange chromatography step removed the major portion of free parotid calmodulin which could compete with calmodulin-coupled Sepharose for the proteins regulated by calmodulin. Thus, by employing an initial chromatography step over DEAE-cellulose to separate phosphodiesterase activity from calmodulin, it was possible to increase the recovery of calmodulin-sensitive phosphodiesterase after affinity chromatography with calmodulin coupled to Sepharose. This approach should be useful for demonstrating the presence of and for purifying other parotid proteins regulated by calmodulin.  相似文献   
9.
Several LINE-1s have been isolated and characterized from genomic DNA of the vole, Microtus kirgisorum. Blot hybridization revealed specific restriction patterns of L1 elements in vole genomes. Rehybridization of the genomic blot with a cloned 5′-end fragment revealed two major bands indicating the presence of two different L1 subfamilies. The copy numbers are estimated for different parts of M. kirgisorum L1 elements. Data also demonstrate that most vole L1 elements are truncated at the 5′-end; however, in contrast to mouse, the ORF1 copy number is higher in vole. A difference between the substitution rates of the ORF1 5′-region (approximately 330 nucleotides) and the rest of the L1 coding regions is revealed. Received: 12 January, 1999 / Accepted: 18 March, 1999  相似文献   
10.
Abstract

Oxidation of 1a with DMSO-acetic or trifluoroacetic anhydride led to the formation of a mixture of epimeric gem-diols 2 and 3 in the ratios of ?3:1 and ?7:1, respectively. Reduction of this mixture with NaBH4 followed by benzoylation and chromatography afforded 4a and 5a.  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