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排序方式: 共有76条查询结果,搜索用时 15 毫秒
1.
Mitre C. Dourado Fábio Protti Jayme L. Szwarcfiter 《Discrete Applied Mathematics》2007,155(18):2435-2443
Let p?1 and q?0 be integers. A family of sets F is (p,q)-intersecting when every subfamily F′⊆F formed by p or less members has total intersection of cardinality at least q. A family of sets F is (p,q)-Helly when every (p,q)-intersecting subfamily F′⊆F has total intersection of cardinality at least q. A graph G is a (p,q)-clique-Helly graph when its family of (maximal) cliques is (p,q)-Helly. According to this terminology, the usual Helly property and the clique-Helly graphs correspond to the case p=2,q=1. In this work we present a characterization for (p,q)-clique-Helly graphs. For fixed p,q, this characterization leads to a polynomial-time recognition algorithm. When p or q is not fixed, it is shown that the recognition of (p,q)-clique-Helly graphs is NP-hard. 相似文献
2.
The role of vanadium bromoperoxidase in the biosynthesis of halogenated marine natural products 总被引:7,自引:0,他引:7
Halogenated natural products are frequently reported metabolites in marine seaweeds. These compounds span a range from halogenated indoles, terpenes, acetogenins, phenols, etc., to volatile halogenated hydrocarbons that are produced on a very large scale. In many cases these halogenated marine metabolites possess biological activities of pharmacological interest. Given the abundance of halogenated marine natural products found in marine organisms and their potentially important biological activities, the biogenesis of these compounds has intrigued marine natural product chemists for decades. Over a quarter of a century ago, a possible role for haloperoxidase enzymes was first suggested in the biogenesis of certain halogenated marine natural products, although this was long before haloperoxidases were discovered in marine organisms. Since that time, FeHeme- and Vanadium-haloperoxidases (V-HPO) have been discovered in many marine organisms. The structure and catalytic activity of vanadium haloperoxidases is reviewed herein, including the importance of V-HPO-catalyzed bromination and cyclization of terpene substrates. 相似文献
3.
For n∈N and D⊆N, the distance graph has vertex set {0,1,…,n−1} and edge set {ij∣0≤i,j≤n−1,|j−i|∈D}. Note that the important and very well-studied circulant graphs coincide with the regular distance graphs.A fundamental result concerning circulant graphs is that for these graphs, a simple greatest common divisor condition, their connectivity, and the existence of a Hamiltonian cycle are all equivalent. Our main result suitably extends this equivalence to distance graphs. We prove that for a finite set D of order at least 2, there is a constant cD such that the greatest common divisor of the integers in D is 1 if and only if for every n, has a component of order at least n−cD if and only if for every n≥cD+3, has a cycle of order at least n−cD. Furthermore, we discuss some consequences and variants of this result. 相似文献
4.
5.
Stefano De Leo Waldyr A. Rodrigues Jayme Vaz 《International Journal of Theoretical Physics》1998,37(9):2415-2431
Complex geometry represents a fundamentalingredient in the formulation of the Dirac equation bythe Clifford algebra. The choice of appropriate complexgeometries is strictly related to the geometricinterpretation of the complex imaginary unit
. We discuss two possibilities which appearin the multivector algebra approach: the123 and 21 complexgeometries. Our formalism provides a set of rules which allows an immediate translation between thecomplex standard Dirac theory and its version withingeometric algebra. The problem concerning a doublegeometric interpretation for the complex imaginary unit
is also discussed. 相似文献
6.
On Clique-Transversals and Clique-Independent Sets 总被引:1,自引:0,他引:1
Guillermo Durán Min Chih Lin Jayme L. Szwarcfiter 《Annals of Operations Research》2002,116(1-4):71-77
A clique-transversal of a graph G is a subset of vertices intersecting all the cliques of G. A clique-independent set is a subset of pairwise disjoint cliques of G. Denote by
C
(G) and
C
(G) the cardinalities of the minimum clique-transversal and maximum clique-independent set of G, respectively. Say that G is clique-perfect when
C
(H)=
C
(H), for every induced subgraph H of G. In this paper, we prove that every graph not containing a 4-wheel nor a 3-fan as induced subgraphs and such that every odd cycle of length greater than 3 has a short chord is clique-perfect. The proof leads to polynomial time algorithms for finding the parameters
C
(G) and
C
(G), for graphs belonging to this class. In addition, we prove that to decide whether or not a given subset of vertices of a graph is a clique-transversal is Co-NP-Complete. The complexity of this problem has been mentioned as unknown in the literature. Finally, we describe a family of highly clique-imperfect graphs, that is, a family of graphs G whose difference
C
(G)–
C
(G) is arbitrarily large. 相似文献
7.
Guillermo Durán Min Chih Lin Sergio Mera Jayme L. Szwarcfiter 《Annals of Operations Research》2008,157(1):37-45
A clique-transversal of a graph G is a subset of vertices intersecting all the cliques of G. It is NP-hard to determine the minimum cardinality τ
c
of a clique-transversal of G. In this work, first we propose an algorithm for determining this parameter for a general graph, which runs in polynomial
time, for fixed τ
c
. This algorithm is employed for finding the minimum cardinality clique-transversal of
[`(3K2)]\overline{3K_{2}}
-free circular-arc graphs in O(n
4) time. Further we describe an algorithm for determining τ
c
of a Helly circular-arc graph in O(n) time. This represents an improvement over an existing algorithm by Guruswami and Pandu Rangan which requires O(n
2) time. Finally, the last proposed algorithm is modified, so as to solve the weighted version of the corresponding problem,
in O(n
2) time. 相似文献
8.
Let G be a graph. If u,v∈V(G), a u-vshortest path of G is a path linking u and v with minimum number of edges. The closed interval I[u,v] consists of all vertices lying in some u-v shortest path of G. For S⊆V(G), the set I[S] is the union of all sets I[u,v] for u,v∈S. We say that S is a convex set if I[S]=S. The convex hull of S, denoted Ih[S], is the smallest convex set containing S. A set S is a hull set of G if Ih[S]=V(G). The cardinality of a minimum hull set of G is the hull number of G, denoted by hn(G). In this work we prove that deciding whether hn(G)≤k is NP-complete.We also present polynomial-time algorithms for computing hn(G) when G is a unit interval graph, a cograph or a split graph. 相似文献
9.
10.
Jayme Vaz Jr. Waldyr A. Rodrigues Jr. 《International Journal of Theoretical Physics》1993,32(6):945-959
In this paper we present an analysis of the possible equivalence of Dirac and Maxwell equations using the Clifford bundle formalism and compare it with Campolattaro's approach, which uses the traditional tensor calculus and the standard Dirac covariant spinor field. We show that Campolattaro's intricate calculations can be proved in few lines in our formalism. We briefly discuss the implications of our findings for the interpretation of quantum mechanics. 相似文献