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   86565篇
  免费   594篇
  国内免费   416篇
化学   29459篇
晶体学   864篇
力学   6862篇
数学   33704篇
物理学   16686篇
  2023年   56篇
  2022年   69篇
  2021年   122篇
  2020年   129篇
  2019年   129篇
  2018年   10542篇
  2017年   10365篇
  2016年   6290篇
  2015年   1086篇
  2014年   583篇
  2013年   871篇
  2012年   4274篇
  2011年   10973篇
  2010年   5983篇
  2009年   6363篇
  2008年   7010篇
  2007年   9181篇
  2006年   667篇
  2005年   1685篇
  2004年   1869篇
  2003年   2222篇
  2002年   1266篇
  2001年   395篇
  2000年   456篇
  1999年   278篇
  1998年   300篇
  1997年   237篇
  1996年   300篇
  1995年   179篇
  1994年   156篇
  1993年   166篇
  1992年   131篇
  1991年   140篇
  1990年   116篇
  1989年   137篇
  1988年   120篇
  1987年   118篇
  1986年   116篇
  1985年   104篇
  1984年   107篇
  1983年   83篇
  1982年   93篇
  1981年   102篇
  1980年   78篇
  1979年   99篇
  1978年   87篇
  1977年   61篇
  1976年   74篇
  1974年   55篇
  1973年   73篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
Central European Journal of Operations Research - The paper discusses operations research (OR) history and achievements in Czechia and Slovakia in 2010–2021 including the most significant OR...  相似文献   
992.
LetK be a convex domain. A maximal snake of sizen is a set of non-overlapping translatesK 1, ...,K N ofK, whereK i touchesK j if and only if |ij|=1 and no translate ofK can touchK 1 orK n without intersecting an additionalK i ,i=1, ...,n. The size of the smallest maximal snake is proved to be 11 ifK is a parallelogram and to be 10 otherwise.  相似文献   
993.
We consider a periodic-review inventory replenishment model with an order-up-to-R operating doctrine for the case of deterministic lead times and a covariance-stationary stochastic demand process. A method is derived for setting the inventory safety stock to achieve an exact desired stockout probability when the autocovariance function for Gaussian demand is known. Because the method does not require that parametric time-series models be fit to the data, it is easily implemented in practice. Moreover, the method is shown to be asymptotically valid when the autocovariance function of demand is estimated from historical data. The effects on the stockout rate of various levels of autocorrelated demand are demonstrated for situations in which autocorrelation in demand goes undetected or is ignored by the inventory manager. Similarly, the changes to the required level of safety stock are demonstrated for varying levels of autocorrelation.  相似文献   
994.
This paper studies the optimal operation of an M/E k /1 queueing system with a removable service station under steady-state conditions. Analytic closed-form solutions of the controllable M/E k /1 queueing system are derived. This is a generalization of the controllable M/M/1, the ordinary M/E k /1, and the ordinary M/M/1 queueing systems in the literature. We prove that the probability that the service station is busy in the steady-state is equal to the traffic intensity. Following the construction of the expected cost function per unit time, we determine the optimal operating policy at minimum cost.  相似文献   
995.
Chiu, in 1976, studied the economic design of np charts for processes subject to a multiplicity of assignable causes. Chiu developed a two-step minimization procedure to obtain the three control variables, namely, the sampling interval, the sample size and the acceptance number. Although Chiu's two-step minimization procedure is simple, applicable and down to earth, it is rather inefficient and has some drawbacks. This paper presents an algorithm to overcome these drawbacks. The results and the execution times of all numerical examples show that the accuracy and the efficiency of the algorithm is reasonably good.  相似文献   
996.
We consider a scheduling problem where a set of n jobs has to be processed on a set of m machines and arbitrary precedence constraints between operations are given. Moreover, for any two operations i and j values a ij >0 and a ji >0 may be given where a ij is the minimal difference between the starting times of operations i and j when operation i is processed first. Often, the objective is to minimize the makespan but we consider also arbitrary regular criteria. Even the special cases of the classical job shop problem J//Cmax belong to the set of NP-hard problems. Therefore, approximation or heuristic algorithms are necessary to handle large-dimension problems. Based on the mixed graph model we give a heuristic decomposition algorithm for such a problem, i.e. the initial problem is partitioned into subproblems that can be solved exactly or approximately with a small error bound. These subproblems are obtained by a relaxation of a subset of the set of undirected edges of the mixed graph. The subproblems are successively solved and a proportion of the results obtained for one subproblem is kept for further subproblem definitions. Numerical results of the algorithm presented here are given.  相似文献   
997.
We develop an exact closed-form expression for the steady-state availability of a repairable, N-server system in which the ith server contains n i identical, reconfigurable, breakdown-prone units. Our approach, which follows from the Markov chain balance equations and the recursive properties of Hessenberg matrix determinants, is simpler than previously proposed matrix geometric approaches, and can readily be adapted to the availability analysis of more complicated structures. We illustrate this by computing the steady-state availability of a mixed parallel-serial gracefully degrading replicated system.  相似文献   
998.
Consider a smooth Hamiltonian system in 2N , , the energy surface ={x/H(x)=H(0)} being compact, and 0 being a hyperbolic equilibrium. We assume, moreover, that {0} is of restricted contact type. These conditions are symplectically invariant. By a variational method, we prove the existence of an orbit homoclinic, i.e. non-constant and doubly asymptotic, to 0.  相似文献   
999.
Recently B.Y. CHEN initiated the study of the tensor product immersion of two immersions of a given Riemannian manifold [3]. In [6] the particular case of tensor product of two Euclidean plane curves was studied. The minimal one were classified, and necessary and sufficient conditions for such a tensor product to be totally real or complex or slant were established. In the present paper we study for tensor product of Euclidean plane curves the problem of B.Y. CHEN: to what extent do the properties of the tensor product immersion f ? h of two immersions f, h determines the immersions f, h ? [3]  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