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   467588篇
  免费   3567篇
  国内免费   1207篇
化学   222516篇
晶体学   6529篇
力学   25215篇
综合类   11篇
数学   66212篇
物理学   151879篇
  2020年   3457篇
  2019年   4121篇
  2018年   12073篇
  2017年   12557篇
  2016年   10855篇
  2015年   4918篇
  2014年   7535篇
  2013年   17251篇
  2012年   16370篇
  2011年   25254篇
  2010年   17197篇
  2009年   17411篇
  2008年   22740篇
  2007年   25432篇
  2006年   13664篇
  2005年   16730篇
  2004年   13771篇
  2003年   12878篇
  2002年   11290篇
  2001年   11642篇
  2000年   8990篇
  1999年   6779篇
  1998年   5719篇
  1997年   5720篇
  1996年   5503篇
  1995年   4913篇
  1994年   4933篇
  1993年   4884篇
  1992年   5294篇
  1991年   5498篇
  1990年   5209篇
  1989年   5227篇
  1988年   5076篇
  1987年   4964篇
  1986年   4734篇
  1985年   6268篇
  1984年   6551篇
  1983年   5286篇
  1982年   5510篇
  1981年   5405篇
  1980年   5152篇
  1979年   5585篇
  1978年   5637篇
  1977年   5647篇
  1976年   5777篇
  1975年   5287篇
  1974年   5258篇
  1973年   5428篇
  1972年   3943篇
  1971年   3421篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
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   
992.
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   
993.
A laboratory practical examination was used to compare the investigative skills developed in two different types of general‐chemistry laboratory courses. Science and engineering majors (SEM) in the control group used a traditional verification approach (SEM‐Ctrl), whereas those in the treatment group learned from an innovative, inquiry‐based approach (SEM‐Trt). A scoring rubric was developed from their examination sheets to assess six component investigative skills. Results indicated that SEM students in the SEM‐Trt group scored significantly higher than those in SEM‐Ctrl for all six skills. Furthermore, nursing and applied science majors (NonSEM) in the inquiry‐based group (NonSEM‐Trt) wrote significantly better discussions than did SEM students in SEM‐Ctrl group. Overall, competency at the mid‐range level of laboratory skills was attained by most SEM‐Trt students (72.5%) but by only 30.5% of SEM‐Ctrl and 28.6% of NonSEM‐Trt students. Apparently, during the semester students in the SEM‐Trt group were able to use the inquiry‐based method effectively to combine chemical tasks with writing tasks and postlaboratory discussions. One implication of this study for science instructors is that practical examinations can provide useful feedback regarding the quality of the laboratory experience. Another implication is that this study provides evidence for the use of the innovative inquiry‐based laboratory approach to support student learning of high‐level investigative skills. However, students' requisite background knowledge must match the level of these skills.  相似文献   
994.
In this paper, we present a new modelling approach for realistic supply chain simulation. The model provides an experimental environment for informed comparison between different supply chain policies. A basic simulation model for a generic node, from which a supply chain network can be built, has been developed using an object-oriented approach. This generic model allows the incorporation of the information and physical systems and decision-making policies used by each node. The object-oriented approach gives the flexibility in specifying the supply chain configuration and operation decisions, and policies. Stochastic simulations are achieved by applying Latin Supercube Sampling to the uncertain variables in descending order of importance, which reduces the number of simulations required. We also present a case study to show that the model is applicable to a real-life situation for dynamic stochastic studies.  相似文献   
995.
996.
Tracing is a method of assigning flows in an electricity network to particular generators and loads, assuming perfect mixing at each node. It can be used to assign costs to transmission users. We show that the resulting allocation is equal to the Shapley value of an equivalent co-operative game.  相似文献   
997.
998.
We construct the polynomial quantization on the space G/H where G=SL(n,R),H=GL(n–1,R). It is a variant of quantization in the spirit of Berezin. In our case covariant and contravariant symbols are polynomials on G/H. We introduce a multiplication of covariant symbols, establish the correspondence principle, study transformations of symbols (the Berezin transform) and of operators. We write a full asymptotic decomposition of the Berezin transform.  相似文献   
999.
Gauss Algebras     
A special class of associative algebras is introduced and studied. A criteria of semisimplicity for suitable category of modules is obtained.  相似文献   
1000.
An algebraic approach to harmonic analysis on reductive Lie groups is proposed. The case of complex semisimple Lie groups is considered in details. Some problems for real reductive Lie groups are discussed.  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