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   30866篇
  免费   84篇
  国内免费   81篇
系统科学   294篇
丛书文集   515篇
教育与普及   84篇
理论与方法论   125篇
现状及发展   12850篇
研究方法   1169篇
综合类   15386篇
自然研究   608篇
  2013年   188篇
  2012年   400篇
  2011年   1049篇
  2010年   161篇
  2008年   478篇
  2007年   526篇
  2006年   563篇
  2005年   552篇
  2004年   592篇
  2003年   552篇
  2002年   492篇
  2001年   901篇
  2000年   910篇
  1999年   540篇
  1992年   551篇
  1991年   469篇
  1990年   498篇
  1989年   454篇
  1988年   483篇
  1987年   493篇
  1986年   454篇
  1985年   651篇
  1984年   493篇
  1983年   398篇
  1982年   325篇
  1981年   346篇
  1980年   434篇
  1979年   929篇
  1978年   767篇
  1977年   732篇
  1976年   630篇
  1975年   696篇
  1974年   883篇
  1973年   797篇
  1972年   772篇
  1971年   948篇
  1970年   1295篇
  1969年   955篇
  1968年   862篇
  1967年   903篇
  1966年   876篇
  1965年   607篇
  1964年   198篇
  1959年   347篇
  1958年   564篇
  1957年   398篇
  1956年   372篇
  1955年   305篇
  1954年   332篇
  1948年   275篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
41.
42.
43.
Summary Traces of nor-adrenaline restore the vascular action of adrenaline altered in epinephrectomized dogs to the reaction of the normal animal. Therefore it is claimed that the adrenals discharge one or several substances into the blood stream, which are necessary for the usual peripheral vascular action of adrenaline. Further investigations are in progress.  相似文献   
44.
45.
Summary The application of diphenylilacetic acid has shown favorable effects on the growth of tobacco plants. Solutions containing 10 p.p.m. and 20 p.p.m. produce, in weight, an almost double growth in treated plants in comparison with the controls. Diphenylilacetic acid can be considered as a growth-promoting substance. The data are not definite.  相似文献   
46.
In econometrics, as a rule, the same data set is used to select the model and, conditional on the selected model, to forecast. However, one typically reports the properties of the (conditional) forecast, ignoring the fact that its properties are affected by the model selection (pretesting). This is wrong, and in this paper we show that the error can be substantial. We obtain explicit expressions for this error. To illustrate the theory we consider a regression approach to stock market forecasting, and show that the standard predictions ignoring pretesting are much less robust than naive econometrics might suggest. We also propose a forecast procedure based on the ‘neutral Laplace estimator’, which leads to an improvement over standard model selection procedures. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
47.
This paper assesses the information content of two survey indicators for consumption developments in the near future for eight European countries in the period 1985–1998. Empirical work on this topic typically focuses on consumer confidence, the perceptions of buyers of consumption goods. This paper examines whether perceptions of sellers of consumption goods, measured by retail trade surveys, may also improve short‐term monitoring of consumption. We find that both consumer confidence and retailer confidence embody valuable information, when analysed in isolation. For France, Italy and Spain we conclude that adding retail confidence does not improve the indicator model once consumer confidence has been included. For the UK the reverse case is obtained. For the remaining four countries we show that combining consumer sentiment and retail trade confidence into a composite indicator leads to optimal results. Our results suggest that incorporating information from retail trade surveys may offer significant benefits for the analysis of short‐term prospects of consumption. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
48.
49.
50.
Obese postmenopausal female volunteers were given timed daily oral dosages of bromocriptine, and tested for reduction of body fat stores. This dopamine agonist has been shown to reset circadian rhythms that are altered in obese animals and to reduce body fat levels in several animal models. The participants were instructed not to alter their existing exercise and eating behavior during treatment. Skinfold measurements were taken on 33 subjects as indices of body fat. The measurements (e.g., suprailiac) were reduced after six weeks by about 25%, which represents a reduction of 11.7% of the total body fat. These dramatic decreases in body fat, which are equivalent to that produced by severe caloric restriction, were accompanied by more modest reductions of body weight (2.5%), indicating a possible conservation of protein that is usually lost as a consequence of such caloric restriction. The effects of bromocriptine treatment on body fat and hyperglycemia were also examined in non-insulin dependent diabetics being treated with oral hypoglycemics (7 subjects) or insulin (7 subjects). Total body fat was reduced by 10.7% and 5.1% in diabetics on oral hypoglycemics and insulin, respectively, without any significant reductions in body weight. Hyperglycemia was reduced in most of the 15 diabetic subjects treated leading to euglycemia and even cessation of hypoglycemic drugs in 3 of the 7 subjects during 4-8 weeks of bromocriptine treatment. These findings support the hypothesis that obesity and type II diabetes may be treated effectively with bromocriptine when administered at the proper times and dosages.  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