全文获取类型
收费全文 | 577篇 |
免费 | 34篇 |
国内免费 | 64篇 |
专业分类
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 |
出版年
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 15 毫秒
111.
Philippe Galinier 《Discrete Applied Mathematics》2007,155(3):312-326
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.
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.
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:A→Q 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