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   1545008篇
  免费   32181篇
  国内免费   8394篇
化学   680619篇
晶体学   20109篇
力学   74723篇
综合类   105篇
数学   241110篇
物理学   372290篇
无线电   196627篇
  2021年   14813篇
  2020年   17386篇
  2019年   17551篇
  2016年   29554篇
  2015年   21739篇
  2014年   32864篇
  2013年   78781篇
  2012年   40938篇
  2011年   37677篇
  2010年   39385篇
  2009年   44193篇
  2008年   40959篇
  2007年   38031篇
  2006年   45658篇
  2005年   36945篇
  2004年   38527篇
  2003年   36154篇
  2002年   37234篇
  2001年   37469篇
  2000年   32839篇
  1999年   29510篇
  1998年   27691篇
  1997年   27504篇
  1996年   26959篇
  1995年   24726篇
  1994年   24209篇
  1993年   23597篇
  1992年   23693篇
  1991年   23799篇
  1990年   22581篇
  1989年   22190篇
  1988年   21342篇
  1987年   20079篇
  1986年   18935篇
  1985年   25409篇
  1984年   26475篇
  1983年   22372篇
  1982年   23743篇
  1981年   22937篇
  1980年   22173篇
  1979年   22210篇
  1978年   23397篇
  1977年   23028篇
  1976年   22630篇
  1975年   21354篇
  1974年   20945篇
  1973年   21501篇
  1972年   15652篇
  1968年   13362篇
  1967年   13446篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
One obtains a formula for the transformation of a vector with a given covariance matrix. The sum of the squares of the components of the transformed vector is equal to a chi-square type quadratic form, constructed with the aid of the initial vector. Taking into account this property, one finds the components of the Pearson statistic, Pearson-Fisher statistic, etc.Translated from Veroyatnostnye Raspredeleniya i Matematicheskaya Statistika, pp. 337–350, 1986.  相似文献   
993.
994.
A convenient method is proposed for the synthesis of all the individual methyl ethers of methyl 2-acetamido-2-deoxy-α-D-glucopyranoside based on the partial methylation of 2-acetamido-2-deoxy-α-D-glucopyranoside with dimethyl sulfate in an alkaline medium followed by preparative liquid column chromatography on silica gel of the resulting mixture of methyl ethers.  相似文献   
995.
Supposa that f(x) is a quasidifferentiable function, defined on S(?)R~n where S is an open set, with a -equivalent bounded quasidifferential subfam ily. Lenma I  相似文献   
996.
Transport as a consequence of state-dependent diffusion   总被引:1,自引:0,他引:1  
Overdamped particles subject to a drift in a force field with sinusoidal space dependence and also a sinusoidally modulated space-dependent diffusion, with the same period as the drift, experience a net driving force. The resulting current depends on the amplitude of the modulation of the diffusion and is a periodic function of the phase difference between the sinusoidal drift and the sinusoidal modulation of the diffusion. For small modulation amplitudes a particle subject to state-dependent noise behaves the same way as a particle subject to thermal noise but with a drift which, in addition to the sinusoidal term, contains a net force term.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday  相似文献   
997.
A narrow linewidth has been obtained in a new 1.5 ?m distributed feedback laser integrated monolithically with a tunable external cavity. The linewidth of 18 MHz has been achieved by tuning the current in the external cavity, and the FM response was flat from 100 kHz to 500 MHz.  相似文献   
998.
Modeling and CW operation of a quasi-three-level 946 nm Nd: YAG laser   总被引:18,自引:0,他引:18  
A model is developed for an end-pumped quasi-three-level laser with population in the lower laser level at equilibrium such as for transitions to the manifolds4I9/2in Nd3+,4I15/2in Er3+,5I8in Ho3+and3H6in Tm3+. It is shown that the effect of residual lower laser level population on laser operation can be treated as a saturable loss. Room temperature operation on the4F3/2-4I9/2transition in Nd:YAG under CW dye laser pumping has been demonstrated with a threshold as low as 11.5 mW incident power and a slope efficiency of 7 percent with 0.3 percent output coupling. Performance is limited by the low output coupling and diffraction loss.  相似文献   
999.
High-resolution positive and negative photoresists were used to fabricate ?/4-phase-shifted corrugations by holographic exposure. To avoid mixing these photoresists, cyclised polyisoprene was used as a midlayer. The shape and depth of the corrugations on the positive and negative photoresist regions were almost the same. Single-mode operation at the Bragg wavelength was confirmed in lasers with the corrugation.  相似文献   
1000.
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