In a graph G = (V (G), E(G)), a vertex dominates itself and its neighbors. A subset S of V (G) is a double dominating set if every vertex of V (G) is dominated at least twice by the vertices of S. The double domination number of G is the minimum cardinality among all double dominating sets of G. We consider the effects of edge removal on the double domination number of a graph. We give a necess...

Several isomorphism classes of graph coverings of a graph G have been enumerated by many authors. Kwak and Lee (Canad. J. Math. XLII (1990) 747) enumerated the isomorphism classes of graph bundles and those of n-fold coverings with respect to a group of automorphisms of the base graph G which 9x a spanning tree. Hofmeister (Discrete Math. 98 (1991) 175) independently enumerated the isomorphism ...

For a connected graph G of order n, a set S of vertices is called a double geodetic set of G if for each pair of vertices x, y in G there exist vertices u, v ∈ S such that x, y ∈ I[u, v]. The double geodetic number dg(G) is the minimum cardinality of a double geodetic set. Any double godetic of cardinality dg(G) is called dg-set of G. The double geodetic numbers of certain standard graphs are o...

A connected n-chromatic graph G is double-critical if for all the edges xy of G, the graph G−x−y is (n−2)-chromatic. In 1966, Erdős and Lovász conjectured that the only double-critical n-chromatic graph is Kn. This conjecture remains unresolved for n ≥ 6. In this short note, we verify this conjecture for claw-free graphs G of chromatic number 6.

A -dimensional -square matrix is defined and certain properties of such matrices are investigated. Two particular graph constructions involving -dimensional -square matrices are given and the graphs so constructed are called matrix graphs. Properties of matrix graphs are determined and an application of matrix graphs to completely independent critical clique is provided. Some attention is g...

A closed 2-cell embedding of a graph embedded in some surface is an embedding such that each face is bounded by a cycle in the graph. The strong embedding conjecture says that every 2-connected graph has a closed 2-cell embedding in some surface. In this paper, we prove that any 2-connected graph without V8 (the M obius 4-ladder) as a minor has a closed 2-cell embedding in some surface. As a c...

A parallel minor is obtained from a graph by any sequence of edge contractions and parallel edge deletions. We prove that, for any positive integer k, every internally 4-connected graph of sufficiently high order contains a parallel minor isomorphic to a variation of K4,k with a complete graph on the vertices of degree k, the k-partition triple fan with a complete graph on the vertices of degre...

Two efficient computer implemented algorithms are presented -=. :'or explicitly constructing all distinct labelings of a graph G :;ith a set of (not necessarily distinct) labels L , given the _;ymmetry group B of G . Two recursive reductions of the problem ,?_rld a precomputation involving certain orbits of stabilizer subgroups q--e the LAtechniques used by the algorithm. Moreover, for each la3...

A cubic graph H is called a Kotzig graph if H has a circuit double cover consisting of three Hamilton circuits. It was first proved by Goddyn that if a cubic graph G contains a spanning subgraph H which is a subdivision of a Kotzig graph then G has a circuit double cover. A spanning subgraph H of a cubic graph G is called a Kotzig frame if the contracted graph G/H is even and every non-circuit ...

In this paper, we give a sufficient condition for graph choosability, based on Combinatorial Nullstellensatz and a specific property, called “double list colorability”, which means that there is a list assignment for which there are exactly two admissible colorings.