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   33122篇
  免费   88篇
  国内免费   83篇
系统科学   191篇
丛书文集   721篇
教育与普及   82篇
理论与方法论   204篇
现状及发展   14356篇
研究方法   1416篇
综合类   15944篇
自然研究   379篇
  2013年   224篇
  2012年   494篇
  2011年   908篇
  2010年   251篇
  2009年   170篇
  2008年   610篇
  2007年   604篇
  2006年   668篇
  2005年   656篇
  2004年   569篇
  2003年   618篇
  2002年   604篇
  2001年   1005篇
  2000年   937篇
  1999年   622篇
  1992年   593篇
  1991年   480篇
  1990年   504篇
  1989年   510篇
  1988年   509篇
  1987年   512篇
  1986年   506篇
  1985年   621篇
  1984年   510篇
  1983年   426篇
  1982年   359篇
  1981年   356篇
  1980年   466篇
  1979年   996篇
  1978年   871篇
  1977年   884篇
  1976年   597篇
  1975年   652篇
  1974年   965篇
  1973年   815篇
  1972年   851篇
  1971年   1047篇
  1970年   1373篇
  1969年   1039篇
  1968年   979篇
  1967年   1018篇
  1966年   859篇
  1965年   629篇
  1959年   366篇
  1958年   527篇
  1957年   453篇
  1956年   377篇
  1955年   321篇
  1954年   372篇
  1948年   203篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
13.
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   
14.
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   
15.
We recently reported on a linkage study within a Quarter Horse lineage segregating hyperkalaemic periodic paralysis (HYPP), an autosomal dominant condition showing potassium-induced attacks of skeletal muscle paralysis. HYPP co-segregated with the equine adult skeletal muscle sodium channel alpha subunit gene, the same gene that causes human HYPP. We now describe the Phe to Leu mutation in transmembrane domain IVS3 which courses the horse disease. This represents the first application of molecular genetics to an important horse disease, and the data will provide an opportunity for control or eradication of this condition.  相似文献   
16.
Summary The tissue specificity of lactate dehydrogenase (EC 1.1.1.27) in brain and eye of the teleostPhallichthys amates was examined by acrylamide gel electrophoresis. It is suggested that subunit association is a function of gene product accessibility superimposed upon genetic restriction of assembly.  相似文献   
17.
Summary Indoxyl derivatives were detected as minor products among the urinary metabolites of two trial drugs, a benzodiazepine (GP 55 129) and a benzophenone (CGP 11 952). Their structures were elucidated by NMR and mass spectroscopy. Presumably, metabolites containing potential aldehyde functions react spontaneously with endogenous indoxyl. Such derivatives have not hitherto been encountered in drug metabolism.  相似文献   
18.
1.0MeV208Pb离子在非晶Si中的投影射程RP和射程偏差ΔRP作为注量和温度二者的函数用背散射法进行测定.注量的变化范围为5×1013~7×1014cm-2.注入是在室温和t=-120℃下完成的.由由实验所确定的投影射程,射程偏差与注量或温度无关,并且分别等于295和72.2nm.与TRIM86的计算结果相比较,发现RP的偏离为18%,而ΔRP的偏离为36%.RP和ΔRP二者与注量及温度的无关性,排除了所观察到的与TRIM的矛盾是由于注入期间辐射增强扩散或离子束混合效应而引起的解释。  相似文献   
19.
Summary A new bromotyrosine-derived alkaloid with antileukemic activity, purealidin A (5), has been isolated from the Okinawan marine spongePsammaplysilla purea and its chemical structure elucidated on the basis of the spectroscopic data.  相似文献   
20.
H Namdari  E J Bottone 《Experientia》1991,47(5):434-436
Aeromonas caviae isolated from stools of diarrheic formula-fed infants and environmental sources produce acetic acid when grown in glucose broth, which is bactericidal (suicide phenomenon). A. caviae grows anaerobically in a minimal medium or under permissive conditions such as the intestinal tract of formula-fed infants. These isolates adhered to HEp-2 cells and produced a cytotoxic and a cytotonic enterotoxin which underscore their enteropathogenicity.  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