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   27682篇
  免费   3813篇
  国内免费   2084篇
化学   5030篇
晶体学   96篇
力学   3425篇
综合类   563篇
数学   13636篇
物理学   10829篇
  2024年   45篇
  2023年   282篇
  2022年   541篇
  2021年   616篇
  2020年   673篇
  2019年   614篇
  2018年   627篇
  2017年   905篇
  2016年   990篇
  2015年   857篇
  2014年   1526篇
  2013年   1984篇
  2012年   1464篇
  2011年   1819篇
  2010年   1592篇
  2009年   1773篇
  2008年   1967篇
  2007年   2061篇
  2006年   1594篇
  2005年   1385篇
  2004年   1343篇
  2003年   1133篇
  2002年   1054篇
  2001年   858篇
  2000年   789篇
  1999年   741篇
  1998年   633篇
  1997年   564篇
  1996年   483篇
  1995年   476篇
  1994年   319篇
  1993年   272篇
  1992年   206篇
  1991年   222篇
  1990年   156篇
  1989年   103篇
  1988年   127篇
  1987年   94篇
  1986年   90篇
  1985年   100篇
  1984年   77篇
  1983年   55篇
  1982年   58篇
  1981年   65篇
  1980年   62篇
  1979年   61篇
  1978年   38篇
  1977年   32篇
  1976年   13篇
  1974年   9篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
The paper studies the Painlevé VIe equations from the point of view of Hamiltonian nonintegrability. For certain infinite number of points in the parameter space we prove that the equations are not integrable. Our approach uses recent advance in Hamiltonian integrability reducing the problem to higher differential Galois groups as well as the monodromy of dilogarithic functions.   相似文献   
192.
ABSTRACT. An indicator defined as a function of the total water diversion through the Coleambally canal and the potential irrigation demand is selected to represent the sustainability of the irrigation water system in the Coleambally irrigation area, Australia. A simulation procedure using a system dynamics approach was developed to evaluate the indicator. The procedure includes water diversion assessment, potential crop water demand and total gross margin. Three cases of water supply options (surface water, ground water pumping and water trading), two cases of changes in the total agricultural area and three cropping pattern scenarios were simulated to better understand their impact on sustainability. The simulated results indicate that increasing the agricultural area reduces the sustainability of the irrigation system because the demand of water increased despite increase in the gross margin. The scenarios show that imposed water trading and ground water pumping would considerably increase the supply system having a positive impact on the sustainability. The paper concludes that a multi‐objective sustainability indicator taking account of economic and environmental issues could be more useful.  相似文献   
193.
We prove analogs of the Telyakovskii-Pochuev criteria for multipliers of uniform convergence and of convergence in the integral metric for multiplicative systems with bounded generating sequence.  相似文献   
194.
The error on a real quantity Y due to the graduation of the measuring instrument may be asymptotically represented, when the graduation is regular and fines down, by a Dirichlet form on R whose square field operator does not depend on the probability law of Y as soon as this law possesses a continuous density. This feature is related to the “arbitrary functions principle” (Poincaré, Hopf). We give extensions of this property to Rd and to the Wiener space for some approximations of the Brownian motion. This gives new approximations of the Ornstein-Uhlenbeck gradient. These results apply to the discretization of some stochastic differential equations encountered in mechanics.  相似文献   
195.
1引言考虑线性代数方程组A_x=b,A∈R~(n×n)非奇异,x,b∈R~n(1)的求解.当系数矩阵是大型稀疏的正定可对称化矩阵,文[1,2]讨论了一类预对称共轭梯度算法(LRSCG算法是其中之一),这类算法的实质是利用非对称的系数矩阵可对称化的性质,并结合共轭梯度法而构造的一种预处理的共轭梯度法[12,16,17].但非对称的系数  相似文献   
196.
The modified quantum nonlinear Schrodinger equation of ultra-short pulse propagating in fiber is derived using dispersion relation and the Hamiltonian of the transmission system. The derived equation is solved with linearization approximation, and modulation instability is analyzed. The equation is also solved with Hartree approximation. The results indicate that pulse power, loss and self-steeping effect change the critical frequency, the maximum gain and the gain spectrum range, but the third order dispersion has no effect on modulation instability. The expectation value of optical field is average of a set of modified classical solitons, and higher order terms change the amplitude, pulse position and phase of soliton.  相似文献   
197.
Polyethers, which are produced by alcoholysis of oxiranes, are often used as solvents in chemical plants. The spectra of the products can be influenced by the choice of the catalyst (acid or base). The results of the base-catalyzed ethanolysis of 1,2-epoxybutane are displayed. The reaction was investigated calorimetrically by varying the molar ratio between ethanol and 1,2-epoxybutane, the starting temperature and the concentration of the catalyst sodium hydroxide. Additionally, the influence of the used alcohol will be shown in one example. To determine the activation parameters, the experiments were evaluated by the nonlinear software package TA-kin v3.3.  相似文献   
198.
This paper considers an optimal boundary control problem fora hyperbolic system in which constant time lags appear in thestate equation and in the boundary condition simultaneously.Making use of Lion's scheme, necessary and sufficient conditionsof optimality for the Neumann problem are derived.  相似文献   
199.
定位定量检测光电内窥系统   总被引:1,自引:1,他引:0  
本文分析定位定量窥测系统设计中普遍存在的问题,提出一种检测方案,并预示窥测仪器的发展趋势。  相似文献   
200.
蔡希洁  黄关龙 《光学学报》1996,16(3):69-273
讨论了大型激光装置放大自发辐射输出的一般特性,给出了神光装置放大自发辐射特性的测量结果。  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