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   338231篇
  免费   2424篇
  国内免费   892篇
化学   162086篇
晶体学   5036篇
力学   16557篇
综合类   2篇
数学   50590篇
物理学   107276篇
  2020年   1764篇
  2019年   2037篇
  2018年   10974篇
  2017年   11786篇
  2016年   7782篇
  2015年   3437篇
  2014年   4009篇
  2013年   11267篇
  2012年   12248篇
  2011年   22149篇
  2010年   13530篇
  2009年   13499篇
  2008年   19308篇
  2007年   22852篇
  2006年   8552篇
  2005年   14617篇
  2004年   10404篇
  2003年   9530篇
  2002年   7507篇
  2001年   7355篇
  2000年   5660篇
  1999年   4248篇
  1998年   3592篇
  1997年   3593篇
  1996年   3455篇
  1995年   3228篇
  1994年   3029篇
  1993年   3071篇
  1992年   3359篇
  1991年   3401篇
  1990年   3211篇
  1989年   3172篇
  1988年   3245篇
  1987年   3107篇
  1986年   2996篇
  1985年   4062篇
  1984年   4244篇
  1983年   3477篇
  1982年   3828篇
  1981年   3720篇
  1980年   3594篇
  1979年   3643篇
  1978年   3882篇
  1977年   3627篇
  1976年   3864篇
  1975年   3435篇
  1974年   3555篇
  1973年   3898篇
  1972年   2363篇
  1971年   1807篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
    
3-[4-(Azidocarbonyl)]phenylsydnone (2) obtained from 3-(4-hydrazinocarbonyl) phenylsydnone (1) on Curtius rearrangement with alcohols, water and amines afforded the corresponding carbamates (3a-h), 4,4′-(sydnone-3-yl) diphenyl urea (4) and 4-(heterocyclyl)phenyl ureas (5a-l). Compounds (5a-l) on one-pot ring conversion yielded the 1,3,4-oxadiazolin-2-one derivatives (6a-l), which on reaction with N2H4 gave the 4-amino-1,2,4-triazolin-3-ones (7a-l). All these compounds exhibited moderate antimicrobial activity against the few microbes tested. The carbamates have been found to be more toxic against fourth instar larvae ofAedes aegypti, in particular, then-butyl derivative (3e).  相似文献   
92.
By extending the notion of mixed states to functionals acting on the space of observables with diagonal singularity we obtain a well-defined complex spectral decomposition of the time evolution for a quantum decaying system. In this formalism, generalized Gamow states are obtained with well-defined physical properties.  相似文献   
93.
Basic ideas and results which characterize quantum diffusion of defects in quantum crystals like solid helium as a new phenomenon are presented. Quantum effects in such media lead to a delocalization of point defects (vacancies, impurities etc.) and they turn into quasiparticles of a new type—defectons, which are characterized not by their position in the crystal lattice but by their quasimomentum and dispersion law. Defecton-defecton and defecton-phonon scattering are considered and an interpolation formula for the diffusion coefficient valid in all interesting temperature and concentration regions is presented. A comparison with the experimental data is made. Some alternative points of view are discussed in detail and the inconsistency of the Kisvarsanyi-Sullivan theory is shown.  相似文献   
94.
95.
The effect of traffic noise on sleep of young adults in their homes   总被引:1,自引:0,他引:1  
The disturbance of sleep by traffic noise is a major problem area in noise pollution. Extensive laboratory tests using multichannel electroencephalograms (EEG) have been carried out by many workers to determine the general response of people when exposed to noise during sleep. An experimental technique for obtaining results in people's homes has been developed using a simplified one-channel EEG. The responses of six people in the age range 19-24 were measured. A significant correlation was found between sleep disturbance and traffic noise when the noise is expressed in EPNdB. In the home experiments the subjects appeared to be approximately 10 dB less sensitive to noises than laboratory subjects for similar noise exposure. There also appeared to be some adaptation to the noise exposure; however, only one subject was tested for a period of 20 days and was insufficient to give definite results on adaptation.  相似文献   
96.
Network survivability analysis   总被引:1,自引:0,他引:1  
Advances in fiber optic and local area network (LAN) technologies are providing system designers with many new options for interconnecting computer equipment. A recurring problem for military system designers, who are end-users of a LAN standard, is implementing LANs so that the survivability, reliability, maintainability, and availability (SRMA) of a system is maximized. LAN standards working groups are currently making design decisions about topologies, components, and protocols that directly affect the SRMA of the end-user's system. This article focuses on a new analytic technique, called “network survivability analysis,” that provides the mathematical tools needed to compare the survivability of arbitrary network topologies. For background, several network metrics are described and their limitations for LAN standards development identified. Network survivability is defined, and several token-passing ring case examples are analyzed. The primary strength of network survivability analysis is that it can be used to provide a quantitative comparison among LAN topologies without knowledge of the end-user's equipment, systems, or environments.  相似文献   
97.
The variational approach based on the covariant quantization method for the curved Einstein space is applied.  相似文献   
98.
Summary Three weak variants of compactness which lie strictly between compactness and quasicompactness, are introduced. Their basic properties are studied. The interplay with mapping and their direct and inverse preservation under mappings are investigated. In the process three decompositions of compactness are observed.  相似文献   
99.
100.
A mixed problem is considered for a system of partial differential equations modeling the process of adsorption dynamics. An existence and uniqueness theorem is proved for this problem, and the solution properties are investigated. The inverse problem is posed, involving the determination of the system coefficient given additional information about the solution. A uniqueness theorem is proved for the solution of the inverse problem.__________Translated from Prikladnaya Matematika i Informatika, No. 16, pp. 5 – 14, 2004.  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