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   5364篇
  免费   153篇
  国内免费   7篇
化学   2823篇
晶体学   21篇
力学   121篇
数学   648篇
物理学   1911篇
  2021年   46篇
  2020年   61篇
  2019年   55篇
  2018年   51篇
  2017年   61篇
  2016年   151篇
  2015年   102篇
  2014年   124篇
  2013年   240篇
  2012年   236篇
  2011年   303篇
  2010年   148篇
  2009年   131篇
  2008年   238篇
  2007年   224篇
  2006年   191篇
  2005年   185篇
  2004年   141篇
  2003年   134篇
  2002年   91篇
  2001年   94篇
  2000年   84篇
  1999年   75篇
  1998年   51篇
  1997年   72篇
  1996年   70篇
  1995年   74篇
  1994年   57篇
  1993年   71篇
  1992年   81篇
  1991年   57篇
  1990年   55篇
  1989年   51篇
  1988年   51篇
  1987年   52篇
  1986年   48篇
  1985年   60篇
  1984年   59篇
  1983年   50篇
  1982年   59篇
  1981年   61篇
  1980年   44篇
  1979年   58篇
  1978年   55篇
  1977年   47篇
  1976年   50篇
  1975年   59篇
  1964年   44篇
  1962年   43篇
  1956年   45篇
排序方式: 共有5524条查询结果,搜索用时 0 毫秒
31.
Summary The question of the existence of good Markov [good stationary] policies is studied for a general class of Borel [stationary] dynamic programming models. It is shown, for example, that Markov [stationary] policies are uniformly adequate if every transition law is absolutely continuous with respect to a fixed measure [and the reward function is positive or the model satisfies certain compactness and continuity conditions].Research supported by Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 72Research supported by National Science Foundation Grant MCS 8100789  相似文献   
32.
The proton conductivity of three layer-type compounds was studied: H3OTi2NbO7. H2O, HTiNbO5 and HTiTaO5. Measurements were made between 20 °C and 90°C on pellets pressed from crystalline powders and soaked with pure water. The hydrated compound had the highest conductivity, (20°C)-6.3–6.9·10–4 (cm)–1. Acid ions from the exchange process were quite firmly retained in its layer structure and contributed significantly to the measured conductivity. Consistent results were obtained after prolonged washing with water.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday  相似文献   
33.
34.
35.
36.
Th. Eicher  V. Schäfer 《Tetrahedron》1974,30(22):4025-4029
The reaction of the azomethine ylides 2a–c with cyclopropenones 3 and of 2a with methylene cyclopropenes 7 leads via (3+3)-cycloaddition to pyridones-4 5 and 1,4 - dihydro - N - methyl - 4 - methylene - pyridines 8, respectively. The merocyanine systems 8 exhibit marked solvatochromic and thermochromic properties.  相似文献   
37.
The theory of chirality functions described in a previous publication is generalized to allow for chiral ligands. In the earlier theory, all symmetry operations of the molecular frame could be thought of as permutations of the ligands among the sites; in the present work, improper rotations not only permute the ligands, but convert them into mirror images. The group that generates all isomers from a given ordered molecule belonging to a frame with n sites is now the hyperoctahedral group of order 2n n! consisting of all possible combinations of permutations and site reflections. The representation theory of these groups is described, and applied to the problem of constructing qualitatively complete chirality functions, and of deciding which ligand partitions, and which isomer mixtures, are chiral. It is found useful to classify chiral representations of the covering group as ligand specific and class specific. The ligand specific representations describe chiral properties which are common to all frames and arise purely from the chirality of the ligands, while the class specific representations describe the chiral properties of the frame. A number of examples are explicitly worked out.  相似文献   
38.
Zusammenfassung Bei der Einwirkung von elementarem Schwefel auf Phenacyl-sulfenylmorpholid entsteht unter H2S-Abspaltung Phenylglyoxyl-säurethionmorpholid. Durch Einsatz von radioaktivem Schwefel wurde festgestellt, daß diese Umlagerungsreaktion über die Stufe einergem. Dimercaptoverbindung verläuft.
Reaction of elementary sulfur with phenacylsulfenylmorpholide gives phenylglyoxylic acid thionmorpholide while hydrogen sulfide is eliminated. The rearrangement proceeds, as was shown by use of radioactive sulfur, via agem. dimercaptocompound as intermediate.


47. Mitt.:F. Asinger, W. Schäfer undA. Saus, Mh. Chem.96, 1265 (1965).

Teil der DissertationA. Saus, Techn. Hochsch. Aachen, 1964.  相似文献   
39.
During recent years high-performance liquid chromatography has become an excellent tool for the determination of antibiotics in biological fluids. Compared with biological assays, the major benefits of this method are specificity and rapidity. In particular, the determination of biologically inactive metabolites emphasizes that this technique plays an outstanding role for the analysis of antibiotics. This paper describes how the method can be used in the analysis of several antibiotics and demonstrates the efficacy of this method for clinical microbiology. Methods for the determination in biological fluids of acylaminopenicillins (azlocillin, mezlocillin, piperacillin and aspoxicillin), quinolones (ciprofloxacine, norfloxacine and ofloxacine), a penem (imipenem) and a cephalosporin (cefixime) are summarized. Furthermore, their application to in vitro studies and their trial in clinical studies are described.  相似文献   
40.
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