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   30486篇
  免费   847篇
  国内免费   241篇
化学   20553篇
晶体学   152篇
力学   703篇
综合类   1篇
数学   5672篇
物理学   4493篇
  2022年   159篇
  2021年   292篇
  2020年   360篇
  2019年   328篇
  2018年   293篇
  2017年   303篇
  2016年   629篇
  2015年   644篇
  2014年   690篇
  2013年   1507篇
  2012年   1524篇
  2011年   1826篇
  2010年   1007篇
  2009年   854篇
  2008年   1578篇
  2007年   1566篇
  2006年   1636篇
  2005年   1484篇
  2004年   1413篇
  2003年   1165篇
  2002年   1111篇
  2001年   493篇
  2000年   453篇
  1999年   422篇
  1998年   403篇
  1997年   452篇
  1996年   434篇
  1995年   393篇
  1994年   422篇
  1993年   380篇
  1992年   359篇
  1991年   339篇
  1990年   307篇
  1989年   311篇
  1988年   297篇
  1987年   242篇
  1986年   261篇
  1985年   401篇
  1984年   408篇
  1983年   339篇
  1982年   387篇
  1981年   344篇
  1980年   374篇
  1979年   299篇
  1978年   337篇
  1977年   308篇
  1976年   257篇
  1975年   210篇
  1974年   224篇
  1973年   195篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
131.
Symmetry is one of the most important aesthetic criteria in graph drawing because it reveals structure in the graph. To draw graphs symmetrically, we employ two steps. The first step is to find appropriate automorphisms. The second step is to draw the graph to display the automorphisms. Our aim in this paper is to construct maximally symmetric straight line drawings of triconnected planar graphs in linear time. Previously known algorithms run in quadratic time. We show that an algorithm of Fontet can be used to find an embedding in the plane with the maximum number of symmetries, and present a new algorithm for finding a straight line drawing that achieves that maximum. Both algorithms run in linear time.  相似文献   
132.
133.
We consider the M(t)/M(t)/m/m queue, where the arrival rate λ(t) and service rate μ(t) are arbitrary (smooth) functions of time. Letting pn(t) be the probability that n servers are occupied at time t (0≤ nm, t > 0), we study this distribution asymptotically, for m→∞ with a comparably large arrival rate λ(t) = O(m) (with μ(t) = O(1)). We use singular perturbation techniques to solve the forward equation for pn(t) asymptotically. Particular attention is paid to computing the mean number of occupied servers and the blocking probability pm(t). The analysis involves several different space-time ranges, as well as different initial conditions (we assume that at t = 0 exactly n0 servers are occupied, 0≤ n0m). Numerical studies back up the asymptotic analysis. AMS subject classification: 60K25,34E10 Supported in part by NSF grants DMS-99-71656 and DMS-02-02815  相似文献   
134.
Carpene  Ettore  Schaaf  Peter  Ariely  Shmuel  Schnek  Rony 《Hyperfine Interactions》2002,139(1-4):495-499
The steel ASTM A213 P22 is used for superheater outlet header in power plants. During duty cycles lasting over tens of years the temperature is of the order of 545°C and the pressure 125 atm. The microscopic changes in these steels are hard to analyze under working conditions, but they are believed to be responsible for the appearance of creeps in such devices. Investigation of the microscopic modifications will help to predict future failures due to creeps, increasing the reliability and saving a lot of money.We report the investigation of such changes in ASTM A213 P22 steel in use for 30 years as a Superheater header at Haifa power station. The analysis has been carried out using Mössbauer Spectroscopy, X-ray diffraction and optical microscopy. As a result it seems that the morphology and phase change of the carbide phases M3C/M7C3 and M23C6 are the most prominent changes leading to the material failures.  相似文献   
135.
136.
Letn cells be arranged in a ring, or alternatively, in a row. Initially, all cells are unmarked. Sequentially, one of the unmarked cells is chosen at random and marked until, aftern steps, each cell is marked. After thekth cell has been marked the configuration of marked cells defines some number of islands: maximal sets of adjacent marked cells. Let ξ k denote the random number of islands afterk cells have been marked. We give explicit expressions for moments of products of ξ k ’s and for moments of products of 1/ξ k ’s. These are used in a companion paper to prove that if a random graph on the natural number is made by drawing an edge betweeni≧1 andj>i with probabilityλ/j, then the graph is almost surely connected ifλ>1/4 and almost surely disconnected ifλ≦1/4.  相似文献   
137.
138.
139.
Many writers and practitioners have mentioned the difficulties that result from randomness in visual interactive simulation or animated simulation modelling. This paper discusses the use of VIS to model transient system behaviour, and proposes some approaches to stochastic VIS modelling which may be helpful in overcoming the difficulties.  相似文献   
140.
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