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   44157篇
  免费   121篇
  国内免费   149篇
系统科学   231篇
丛书文集   852篇
教育与普及   131篇
理论与方法论   242篇
现状及发展   19802篇
研究方法   1662篇
综合类   20992篇
自然研究   515篇
  2013年   291篇
  2012年   586篇
  2011年   1107篇
  2010年   305篇
  2008年   766篇
  2007年   767篇
  2006年   805篇
  2005年   817篇
  2004年   784篇
  2003年   780篇
  2002年   771篇
  2001年   1248篇
  2000年   1171篇
  1999年   804篇
  1994年   385篇
  1992年   737篇
  1991年   615篇
  1990年   667篇
  1989年   639篇
  1988年   652篇
  1987年   688篇
  1986年   703篇
  1985年   837篇
  1984年   652篇
  1983年   580篇
  1982年   502篇
  1981年   491篇
  1980年   617篇
  1979年   1326篇
  1978年   1154篇
  1977年   1117篇
  1976年   815篇
  1975年   926篇
  1974年   1288篇
  1973年   1107篇
  1972年   1150篇
  1971年   1445篇
  1970年   1800篇
  1969年   1420篇
  1968年   1331篇
  1967年   1331篇
  1966年   1151篇
  1965年   837篇
  1959年   483篇
  1958年   789篇
  1957年   624篇
  1956年   531篇
  1955年   489篇
  1954年   519篇
  1948年   334篇
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
41.
42.
43.
The visual field is topographically mapped onto the primary visual cortex (V1), forming a retinotopic map which is far more detailed for the foveal regions than for the periphery. We found that receptive field (RF) size in monkey V1 increases with eccentricity, and that a 1—2 m2 V1 region contains roughly a complete set of machinery necessary to analyze a visual-field area whose size is about that of the RFs of the cells. This allows V1 to be anatomically uniform, and is in sharp contrast with the retina.  相似文献   
44.
1 Results This paper presents the development of platinum nanocomposites structures based on organically modified c.a.2 nm core platinum nanoparticles.The chemical modification of the 4-mercaptoaniline functionalized particles by various in coming molecules is evidenced and precisely quantified.The particles can be dissolved like molecules in various solvents depending on the features of the new crown and X-rays shows that the interparticle distance is affected by the crown modification.These platinum n...  相似文献   
45.
1 Introduction   Nanoscale science and technology have attracted greatattention since the novel properties become dominant for well-known materials as their sizes reduced to some critical dimension. These properties frequently originate in lattice distortions, structure transformations etc[1]. The variations of size and structure ofnanocrystals lead to the change of their electronic structures. Very recently, nanoscale materials with different electronic structures were utilized in fabricating nanoscale junctions[2].   ……  相似文献   
46.
煤矸石空心砖是一种新型环保墙体材料,符合国家及山西省出台的限制生产和使用实心黏土砖,鼓励发展新型建筑材料的政策。利用煤矸石制造的空心砖可以作为保温材料,既可节约建筑能耗,又可节约土地资源,减少环境污染。煤矸石空心砖同黏土实心砖相比,具有高强、轻质、隔音、隔热、保温、防震等优点,市场前景良好。  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.
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   
50.
A linear regression model with random walk coefficients is extended to allow for linear restrictions between the coefficients to be satisfied at each point in time. Estimation in this model is shown to be no more involved than estimation in the standard model. It is also demonstrated how, after a slight modification to the testing problem, classical test procedures may be applied to the problem of testing for such restrictions. The performance of the Lagrange Multiplier test for a variety of different restrictions is then investigated via simulation. An empirical application involving testing for homogeneity in a random walk coefficient version of the AIDS model is given.  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