Published online by Cambridge University Press: 18 May 2009
All the graphs considered in this paper are connected finite undirected graphs without loops and multiple edges.
By the vertex-neighbourhood (v-neighbourhood) of any vertex x in the graph G we mean the subgraph induced by the set of all vertices adjacent to x. Analogously by the edge-neighbourhood (e-neighbourhood) of any edge f with end vertices x, y we mean the subgraph (f) (or (xy)) induced by the set of all vertices which are adjacent to at least one vertex of the pair x, y and which are different from x, y.