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   32772篇
  免费   134篇
  国内免费   295篇
系统科学   444篇
丛书文集   538篇
教育与普及   131篇
理论与方法论   118篇
现状及发展   13263篇
研究方法   1241篇
综合类   16807篇
自然研究   659篇
  2013年   357篇
  2012年   638篇
  2011年   1297篇
  2010年   448篇
  2009年   426篇
  2008年   749篇
  2007年   864篇
  2006年   852篇
  2005年   783篇
  2004年   679篇
  2003年   560篇
  2002年   569篇
  2001年   873篇
  2000年   943篇
  1999年   650篇
  1994年   341篇
  1992年   503篇
  1991年   425篇
  1990年   475篇
  1989年   443篇
  1988年   434篇
  1987年   472篇
  1986年   461篇
  1985年   559篇
  1984年   440篇
  1983年   386篇
  1982年   356篇
  1981年   347篇
  1980年   408篇
  1979年   889篇
  1978年   727篇
  1977年   708篇
  1976年   542篇
  1975年   661篇
  1974年   848篇
  1973年   746篇
  1972年   742篇
  1971年   920篇
  1970年   1034篇
  1969年   895篇
  1968年   884篇
  1967年   854篇
  1966年   757篇
  1965年   552篇
  1959年   305篇
  1958年   526篇
  1957年   409篇
  1956年   324篇
  1955年   336篇
  1954年   313篇
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
1.
String theorists are certain that they are practicing physicists. Yet, some of their recent critics deny this. This paper argues that this conflict is really about who holds authority in making rational judgment in theoretical physics. At bottom, the conflict centers on the question: who is a proper physicist? To illustrate and understand the differing opinions about proper practice and identity, we discuss different appreciations of epistemic virtues and explanation among string theorists and their critics, and how these have been sourced in accounts of Einstein's biography. Just as Einstein is claimed by both sides, historiography offers examples of both successful and unsuccessful non-empirical science. History of science also teaches that times of conflict are often times of innovation, in which novel scholarly identities may come into being. At the same time, since the contributions of Thomas Kuhn historians have developed a critical attitude towards formal attempts and methodological recipes for epistemic demarcation and justification of scientific practice. These are now, however, being considered in the debate on non-empirical physics.  相似文献   
2.
Clutch size (CS) is a well-studied life history characteristic, and optimal egg size theory predicts that CS should correlate positively with reproductive investment. Turtles are good subjects for studies of reproductive strategies because few turtle species exhibit parental care; therefore quantifying their short-term reproductive investment is mostly limited to measuring egg size and number. Clutch size is usually reported as an average value for each turtle population, and where CS variation is noted, it is usually attributable to variation in adult body size. In two long-term studies of ecologically dissimilar species we detected a dramatic increase in CS in a common snapping turtle (Chelydra serpentina) population and in a diamondback terrapin (Malaclemys terrapin) population. It is unknown whether these changes are due to either variation in adult body size or resource availability; but the temporal patterns we observed have apparently never been reported previously. These trends remain unexplained and should be explored further.  相似文献   
3.
4.
If the general arguments concerning theinvolvement of variation and selection inexplanations of ``fit'' are valid, then variationand selection explanations should beappropriate, or at least potentiallyappropriate, outside the paradigm historisticdomains of biology and knowledge. In thisdiscussion, I wish to indicate some potentialroles for variation and selection infoundational physics – specifically inquantum field theory. I will not be attemptingany full coherent ontology for quantum fieldtheory – none currently exists, and none islikely for at least the short term future. Instead, I wish to engage in some partiallyspeculative interpretations of some interestingresults in this area with the aim ofdemonstrating that variation and selectionnotions might play a role even here. Ifvariation and selection can survive in even asinhospitable and non-paradigmatic a terrain asfoundational physics, then it can surviveanywhere.  相似文献   
5.
对矿井索车优化设计中的几个关键性问题进行了探讨,包括驱动轮和尾轮的设计、钢丝绳直径的选择计算、托绳轮间距的确定、绳卡的设计、张紧力和张紧行程的确定、互锁问题的解决等。  相似文献   
6.
西文期刊回溯建库中几个重要项目的著录   总被引:4,自引:0,他引:4  
在实践的基础上,从期刊的出版日期、出版周期、卷、期、年、月及其他标识、ISSN、增刊、出版者以及期刊的历史发展等方面对西文期刊机读目录的标准化著录问题进行了探讨。  相似文献   
7.
关于21世纪图书馆员继续教育问题的思考   总被引:4,自引:1,他引:3  
论述了21世纪对图书馆员进行继续教育的必要性,介绍了图书馆员继续教育的内容,针对我国图书馆员继续教育的状况,提出了应采取加大投入、进行针对性培训、运用激励机制等措施来加强对图书馆员的继续教育。  相似文献   
8.
B S Qiu  C H Cho  C W Ogle 《Experientia》1992,48(4):389-391
Ten-day treatment with nicotine (5, 25 or 50 micrograms/ml drinking water) dose-dependently intensified gastric ulceration induced by cold-restraint, and emptying rate. Stomach contractions produced by graded doses of bethanechol i.v. were elevated further by nicotine treatment. It is suggested that chronic nicotine administration produces hypersensitivity of the gastric muscarinic receptors; stomach hypermotility contributes to the ulcer-worsening action of the alkaloid.  相似文献   
9.
10.
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