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   577篇
  免费   34篇
  国内免费   64篇
化学   10篇
晶体学   1篇
力学   17篇
综合类   13篇
数学   609篇
物理学   25篇
  2022年   9篇
  2021年   10篇
  2020年   6篇
  2019年   16篇
  2018年   15篇
  2017年   8篇
  2016年   15篇
  2015年   8篇
  2014年   20篇
  2013年   36篇
  2012年   29篇
  2011年   17篇
  2010年   32篇
  2009年   29篇
  2008年   42篇
  2007年   36篇
  2006年   46篇
  2005年   26篇
  2004年   30篇
  2003年   31篇
  2002年   25篇
  2001年   24篇
  2000年   27篇
  1999年   25篇
  1998年   32篇
  1997年   19篇
  1996年   16篇
  1995年   3篇
  1994年   4篇
  1993年   8篇
  1992年   1篇
  1991年   1篇
  1990年   3篇
  1989年   8篇
  1988年   3篇
  1987年   3篇
  1986年   4篇
  1985年   2篇
  1984年   2篇
  1982年   1篇
  1980年   2篇
  1979年   1篇
排序方式: 共有675条查询结果,搜索用时 62 毫秒
111.
Given a finite set E and a family F={E1,…,Em} of subsets of E such that F covers E, the famous unicost set covering problem (USCP) is to determine the smallest possible subset of F that also covers E. We study in this paper a variant, called the Large Set Covering Problem (LSCP), which differs from the USCP in that E and the subsets Ei are not given in extension because they are very large sets that are possibly infinite. We propose three exact algorithms for solving the LSCP. Two of them determine minimal covers, while the third one produces minimum covers. Heuristic versions of these algorithms are also proposed and analysed. We then give several procedures for the computation of a lower bound on the minimum size of a cover. We finally present algorithms for finding the largest possible subset of F that does not cover E. We also show that a particular case of the LSCP is to determine irreducible infeasible sets in inconsistent constraint satisfaction problems. All concepts presented in the paper are illustrated on the k-colouring problem which is formulated as a constraint satisfaction problem.  相似文献   
112.
113.
利用Building理论获得一种计算某些三角几何的基本群的新的方法.这种方法能够容易地计算出无限多有限三角几何的基拓扑基本群.  相似文献   
114.
准晶理论的新进展   总被引:2,自引:0,他引:2  
郭可信 《物理》2000,29(12):708-711
准晶的高分辨电子显微像常显示为直径为2nm、呈十重对称的圆盘,它们还经常重叠。据此,Gummelt在1995年设计出一种内部涂有两种颜色的正十边形,并且证明只要在覆盖时两个十边形中同一颜色的部分相重,就会给出准周期的Penrose图。这种覆盖理论只需要一种正十边形的“准单胞”,覆盖部分是相邻“准单胞”生长的核。覆盖越多,结构越稳定。Gummelt在1999年将正十边形中的两种颜色去掉,既可给出准周期结构,也可给出周期结构,以及介于它们两者之间的各种中间结构。  相似文献   
115.
Certain graph‐theoretic properties and alternative definitions of the Gray graph, the smallest known cubic edge‐ but not vertex‐transitive graph, are discussed in detail. © 2000 John Wiley & Sons, Inc. J Graph Theory 35: 1–7, 2000  相似文献   
116.
The minimum number of k-subsets out of a v-set such that each t-set is contained in at least one k-set is denoted by C(v, k, t). In this article, a computer search for finding good such covering designs, leading to new upper bounds on C(v, k, t), is considered. The search is facilitated by predetermining automorphisms of desired covering designs. A stochastic heuristic search (embedded in the general framework of tabu search) is then used to find appropriate sets of orbits. A table of upper bounds on C(v, t + 1, t) for v 28 and t 8 is given, and the new covering designs are listed. © 1999 John Wiley & Sons, Inc. J. Combin Designs 7: 217–226, 1999  相似文献   
117.
A t-(v, k, λ) covering is an incidence structure with v points, each block incident on exactly k points, such that every set of t distinct points is incident on at least λ blocks. By considering certain geometries over finite principal ideal rings, we construct infinite families of t-(v, k, λ) coverings having many interesting combinatorial properties. © 1999 John & Sons, Inc. J Combin Designs 7: 247–268, 1999  相似文献   
118.
廖龙光  付虹  傅秀军 《物理学报》2009,58(10):7088-7093
研究十二次对称准晶体模型的几何结构性质.修正了Socolar在正方形-菱形-六边形拼图模型中自相似变换的错误.在Stampfli-Ghler正方形-菱形-三角形拼图模型的基础上,成功地构造出了准晶胞,使得十二次对称准周期结构可以用覆盖理论描述. 关键词: 准晶体 拼图模型 自相似变换 覆盖理论  相似文献   
119.
The Zhang–Zhang polynomial (i.e., Clar covering polynomial) of hexagonal systems is introduced by H. Zhang and F. Zhang, which can be used to calculate many important invariants such as the Clar number, the number of Kekulé structures and the first Herndon number, etc. In this paper, we give out an explicit recurrence expression for the Zhang–Zhang polynomials of the cyclo-polyphenacenes, and determine their Clar numbers, numbers of Kekulé structures and their first Herndon numbers.  相似文献   
120.
Enumerating the isomorphism classes of several types of graph coverings is one of the central research topics in enumerative topological graph theory (see [R. Feng, J.H. Kwak, J. Kim, J. Lee, Isomorphism classes of concrete graph coverings, SIAM J. Discrete Math. 11 (1998) 265-272; R. Feng, J.H. Kwak, Typical circulant double coverings of a circulant graph, Discrete Math. 277 (2004) 73-85; R. Feng, J.H. Kwak, Y.S. Kwon, Enumerating typical circulant covering projections onto a circulant graph, SIAM J. Discrete Math. 19 (2005) 196-207; SIAM J. Discrete Math. 21 (2007) 548-550 (erratum); M. Hofmeister, Graph covering projections arising from finite vector spaces over finite fields, Discrete Math. 143 (1995) 87-97; M. Hofmeister, Enumeration of concrete regular covering projections, SIAM J. Discrete Math. 8 (1995) 51-61; M. Hofmeister, A note on counting connected graph covering projections, SIAM J. Discrete Math. 11 (1998) 286-292; J.H. Kwak, J. Chun, J. Lee, Enumeration of regular graph coverings having finite abelian covering transformation groups, SIAM J. Discrete Math. 11 (1998) 273-285; J.H. Kwak, J. Lee, Isomorphism classes of graph bundles, Canad. J. Math. XLII (1990) 747-761]). A covering is called abelian (or circulant, respectively) if its covering graph is a Cayley graph on an abelian (or a cyclic, respectively) group. A covering p from a Cayley graph onto another Cay (Q,Y) is called typical if the map p:AQ on the vertex sets is a group epimorphism. Recently, the isomorphism classes of connected typical circulant r-fold coverings of a circulant graph are enumerated in [R. Feng, J.H. Kwak, Typical circulant double coverings of a circulant graph, Discrete Math. 277 (2004) 73-85] for r=2 and in [R. Feng, J.H. Kwak, Y.S. Kwon, Enumerating typical circulant covering projections onto a circulant graph, SIAM J. Discrete Math. 19 (2005) 196-207; SIAM J. Discrete Math. 21 (2007) 548-550 (erratum)] for any r. As a continuation of these works, we enumerate in this paper the isomorphism classes of typical abelian prime-fold coverings of a circulant graph.  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