Results 41 to 50 of about 1,222,002 (199)
Exact Values and Bounds on Covering Schemes of Strength Two
Journal of Combinatorial Designs, EarlyView.ABSTRACT In this work, we investigate covering schemes of strength 2 over finite abelian groups, establishing new lower and upper bounds and evaluating new exact values. A main result is a new general lower bound that improves the trivial one and achieves optimality for the binary case. We also develop a recursive relation based on subsets of the group
André G. Castoldi +4 more
wiley +1 more source
On Odd Covers of Cliques and Disjoint Unions
Journal of Graph Theory, EarlyView.ABSTRACT Babai and Frankl posed the “odd cover problem” of finding the minimum cardinality of a collection of complete bipartite graphs such that every edge of the complete graph of order n $n$ is covered an odd number of times. In a previous paper with O'Neill, some of the authors proved that this value is always ⌈ n / 2 ⌉ $\lceil n/2\rceil $ or ⌈ n /
Calum Buchanan +7 more
wiley +1 more source
Sparse Graphs With Local Covering Conditions on Edges
Journal of Graph Theory, EarlyView.ABSTRACT In 1988, Erdős suggested the question of minimizing the number of edges in a connected n $n$‐vertex graph where every edge is contained in a triangle. Shortly after, Catlin, Grossman, Hobbs, and Lai resolved this in a stronger form. In this paper, we study a natural generalization of the question of Erdős in which we replace “triangle” with ...
Debsoumya Chakraborti +3 more
wiley +1 more source
Crossing number is hard for cubic graphs [PDF]
, 2004 It was proved by [Garey, Johnson] that computing the crossing number of a graph is an NP -hard problem. Their reduction, however, used parallel edges and vertices of very high degrees.
Petr Hlineny +7 more
core +1 more source
A theorem about a conjecture of H. Meyniel on kernel-perfect graphs
Discrete Mathematics, 1986 An R-digraph (also called kernel-perfect graph) is a digraph such that all of its induced subdigraphs possess a kernel (that is an independent dominating subset). Meyniel's conjecture (suggested in the title) is that D is an R-digraph if all odd directed cycles of D possess two pseudodiagonals (a pseudo-diagonal is an arc which is not part of the cycle
openaire +2 more sources
A Note on Lovász Characterization of Perfect Graphs
Journal of Graph Theory, EarlyView.ABSTRACT A graph is perfect if, for every induced subgraph, the chromatic number equals the size of its largest clique. In 1972, Lovász established a fundamental characterization of perfect graphs, showing that a graph is perfect if and only if, for every induced subgraph, the product of the size of the largest independent set and the size of the ...
James Alex
wiley +1 more source
A proof of the four-colour theorem
, 2013 The four-colour problem remained unsolved for more than a hundred years has played a role of the utmost importance in the development of graph theory. The four-colour theorem was confirmed in 1976, which is not completely satisfied due to: i) part of the
Xing, Jing Tang
core +1 more source
Journal of Graph Theory, EarlyView.ABSTRACT A 2‐edge‐coloured graph G $G$ is called locally complete if for each vertex v $v$, the vertices adjacent to v $v$ through edges of the same colour induce a complete subgraph in G $G$. Locally complete 2‐edge‐coloured graphs have nice properties, and there exists a polynomial algorithm to decide whether such a graph has an alternating ...
Jørgen Bang‐Jensen, Jing Huang
wiley +1 more source
The 1 3 $\frac{1}{3}$‐Conjectures for Domination in Cubic Graphs
Journal of Graph Theory, EarlyView.ABSTRACT A set S $S$ of vertices in a graph G $G$ is a dominating set of G $G$ if every vertex not in S $S$ is adjacent to a vertex in S $S$. The domination number of G $G$, denoted by γ ( G ) $\gamma (G)$, is the minimum cardinality among all dominating sets in G $G$. In a breakthrough paper in 2008, Löwenstein and Rautenbach proved that if G $G$ is a
Paul Dorbec, Michael A. Henning
wiley +1 more source
From Finding a Spanning Subgraph H $H$ to an H $H$‐Factor
Journal of Graph Theory, EarlyView.ABSTRACT A typical Dirac‐type problem in extremal graph theory is to determine the minimum degree threshold for a graph G $G$ to have a spanning subgraph H $H$, for example, the Dirac theorem. A natural follow‐up problem is to seek an H $H$‐factor, which is a spanning set of vertex‐disjoint copies of H $H$.
Allan Lo
wiley +1 more source