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   430256篇
  免费   2548篇
  国内免费   881篇
化学   208900篇
晶体学   6075篇
力学   22536篇
综合类   13篇
数学   62879篇
物理学   133282篇
  2020年   3338篇
  2019年   3961篇
  2018年   11787篇
  2017年   12346篇
  2016年   10362篇
  2015年   4341篇
  2014年   6573篇
  2013年   14797篇
  2012年   14395篇
  2011年   22774篇
  2010年   15427篇
  2009年   15726篇
  2008年   20881篇
  2007年   23684篇
  2006年   12157篇
  2005年   15207篇
  2004年   12202篇
  2003年   11670篇
  2002年   10219篇
  2001年   9998篇
  2000年   7916篇
  1999年   5825篇
  1998年   5165篇
  1997年   5045篇
  1996年   4828篇
  1995年   4136篇
  1994年   4147篇
  1993年   4220篇
  1992年   4383篇
  1991年   4691篇
  1990年   4511篇
  1989年   4575篇
  1988年   4418篇
  1987年   4420篇
  1986年   4199篇
  1985年   5382篇
  1984年   5525篇
  1983年   4597篇
  1982年   4925篇
  1981年   4711篇
  1980年   4350篇
  1979年   4813篇
  1978年   5122篇
  1977年   5110篇
  1976年   5195篇
  1975年   4906篇
  1974年   4825篇
  1973年   5133篇
  1972年   3782篇
  1971年   3350篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
For vector quasivariational inequalities involving multifunctions in topological vector spaces, an existence result is obtained without a monotonicity assumption and with a convergence assumption weaker than semicontinuity. A new type of quasivariational inequality is proposed. Applications to quasicomplementarity problems and traffic network equilibria are considered. In particular, definitions of weak and strong Wardrop equilibria are introduced for the case of multivalued cost functions.  相似文献   
972.
Defining speed of diffusion as the amount of time it takes to get from one penetration level to a higher one, we introduce a dynamic model in which we study the link between pricing policy, speed of diffusion, and number of competitors in the market. Our analysis shows that, in the case of strategic (oligopolistic) competition, the speed of diffusion has an important influence on the optimal pricing policy. In particular, we find that higher speeds of diffusion create an incentive to strategically interacting firms to lower their prices.  相似文献   
973.
A computer construction of the Monster is used to prove thatPSL(2, 59) is a subgroup of the Monster.  相似文献   
974.
We shall answer several questions concerning additive or multiplicative commutators in a division ring which are algebraic of bounded degree over its center.  相似文献   
975.
There is growing evidence that temporal lobe seizures are preceded by a preictal transition, characterized by a gradual dynamical change from asymptomatic interictal state to seizure. We herein report the first prospective analysis of the online automated algorithm for detecting the preictal transition in ongoing EEG signals. Such, the algorithm constitutes a seizure warning system. The algorithm estimates STLmax, a measure of the order or disorder of the signal, of EEG signals recorded from individual electrode sites. The optimization techniques were employed to select critical brain electrode sites that exhibit the preictal transition for the warning of epileptic seizures. Specifically, a quadratically constrained quadratic 0-1 programming problem is formulated to identify critical electrode sites. The automated seizure warning algorithm was tested in continuous, long-term EEG recordings obtained from 5 patients with temporal lobe epilepsy. For individual patient, we use the first half of seizures to train the parameter settings, which is evaluated by ROC (Receiver Operating Characteristic) curve analysis. With the best parameter setting, the algorithm applied to all cases predicted an average of 91.7% of seizures with an average false prediction rate of 0.196 per hour. These results indicate that it may be possible to develop automated seizure warning devices for diagnostic and therapeutic purposes.Mathematics Subject Classification (1991):20E28, 20G40, 20C20  相似文献   
976.
Kinetic models for chemotaxis, nonlinearly coupled to a Poisson equation for the chemo-attractant density, are considered. Under suitable assumptions on the turning kernel (including models introduced by Othmer, Dunbar and Alt), convergence in the macroscopic limit to a drift-diffusion model is proven. The drift-diffusion models derived in this way include the classical Keller-Segel model. Furthermore, sufficient conditions for kinetic models are given such that finite-time-blow-up does not occur. Examples are given satisfying these conditions, whereas the macroscopic limit problem is known to exhibit finite-time-blow-up. The main analytical tools are entropy techniques for the macroscopic limit as well as results from potential theory for the control of the chemo-attractant density.Present address: Centro de Matemática e Aplicações Fundamentais, Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003, Lisboa, Portugal  相似文献   
977.
We prove the existence of solutions to nonlinear free boundary problem with singularities at given points.  相似文献   
978.
We analyze the long-time behavior of transport equations for a class of dissipative quantum systems with Fokker-planck type diffusion operator, subject to confining potentials of harmonic oscillator type. We establish the existence and uniqueness of a non-equilibrium steady state for the corresponding dynamics. Further, using a (classical) convex Sobolev inequality, we prove an optimal exponential rate of decay towards this state and additionally give precise dispersion estimates in those cases, where no stationary state exists.  相似文献   
979.
A carousel is a dynamical system that describes the movement of an equilateral linkage in which the midpoint of each rod travels parallel to it. They are closely related to the floating body problem. We prove, using the work of Auerbach, that any figure that floats in equilibrium in every position is drawn by a carousel. Of special interest are such figures with rational perimetral density of the floating chords, which are then drawn by carousels. In particular, we prove that for some perimetral densities the only such figure is the circle, as the problem suggests.  相似文献   
980.
The aim of the paper is to present and substantiate a technique to visualize DEA modelling results without any loss of mathematical rigour. The proposed family of parametric optimization methods allows one to construct an intersection of the multidimensional frontier with a two-dimensional plane determined by any pair of given directions. This approach reduces the efficiency analysis of production units to the investigation of well-known functions in economics. We also propose constructive methods to calculate marginal rates of substitution, marginal rates of transformation and so on.  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