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   51118篇
  免费   144篇
  国内免费   160篇
系统科学   297篇
丛书文集   937篇
教育与普及   129篇
理论与方法论   304篇
现状及发展   22047篇
研究方法   1915篇
综合类   24958篇
自然研究   835篇
  2013年   430篇
  2012年   748篇
  2011年   1590篇
  2010年   373篇
  2008年   915篇
  2007年   970篇
  2006年   1003篇
  2005年   997篇
  2004年   939篇
  2003年   918篇
  2002年   913篇
  2001年   1525篇
  2000年   1427篇
  1999年   941篇
  1992年   865篇
  1991年   713篇
  1990年   732篇
  1989年   759篇
  1988年   746篇
  1987年   742篇
  1986年   757篇
  1985年   931篇
  1984年   761篇
  1983年   623篇
  1982年   568篇
  1981年   540篇
  1980年   678篇
  1979年   1523篇
  1978年   1291篇
  1977年   1255篇
  1976年   946篇
  1975年   1048篇
  1974年   1492篇
  1973年   1296篇
  1972年   1347篇
  1971年   1596篇
  1970年   2058篇
  1969年   1571篇
  1968年   1553篇
  1967年   1531篇
  1966年   1367篇
  1965年   984篇
  1964年   265篇
  1959年   587篇
  1958年   925篇
  1957年   692篇
  1956年   584篇
  1955年   494篇
  1954年   577篇
  1948年   368篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
51.
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   
52.
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   
53.
54.
E Littler  A D Stuart  M S Chee 《Nature》1992,358(6382):160-162
Human cytomegalovirus (HCMV, a betaherpes virus) is the cause of serious disease in immunologically compromised individuals, including those with acquired immunodeficiency syndrome. One of the compounds used in the chemotherapy of HCMV infections is the nucleoside analogue 9-(1,3-dihydroxy-2-propoxymethyl)-guanine (ganciclovir). The mechanism of action of this drug is dependent on the formation of the nucleoside triphosphate, which is a strong inhibitor of the viral DNA polymerase. Thymidine kinase, which is encoded by many of the herpesviruses, catalyses the initial phosphorylation of ganciclovir. But there is no evidence for the coding of this enzyme by HCMV, and DNA sequence analysis of the HCMV genome has shown that there is no open reading frame characteristic of a herpesvirus thymidine kinase. Here we present biochemical and immunological evidence that the HCMV UL97 open reading frame codes for a protein capable of phosphorylating ganciclovir. This protein seems to be responsible for the selectivity of ganciclovir and will be useful tool in the understanding and refinement of the antiviral activity of new selective anti-HCMV compounds.  相似文献   
55.
The characteristic circular dichroism of bilirubin bound to human serum albumin undergoes a remarkable sign inversion on addition of halothane, chloroform and other volatile anesthetics. This sign inversion, which is completely reversed by removal of the anesthetic, reflects a pronounced conformational change of the bound ligand; probably a complete inversion of chirality. The observation suggests that association of volatile anesthetics with proteins can markedly alter the internal topography of receptor sites and potentially influence the stereoselectivity of ligand binding.  相似文献   
56.
本文介绍利用信号流图及梅森公式分析设计负反馈电桥电路,以及在电阻传感器测量线性化中的应用。  相似文献   
57.
58.
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   
59.
DNA fingerprinting transforms the art of cell authentication.   总被引:4,自引:0,他引:4  
G N Stacey  B J Bolton  A Doyle 《Nature》1992,357(6375):261-262
The increasing diversity of new cell cultures is seriously stretching the capabilities of traditional methods of identification. DNA fingerprinting is set to play an important role in increasing confidence in the authenticity of cultures in research and industry.  相似文献   
60.
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   
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