i. Introduction. A graph G is an ordered pair (V, E) where V is a set of objects called vertices, and E is a set of unordered pairs of vertices (v,v f ) in which each such pair can occur at most once in E, and if (v,v f ) c E then v 4 v 1 . The order of G is the cardinality of the set V, and the degree 6(v) of an element v € V is the number of elements of E which contain v. G is said to be regular of degree d if 5(v) = d for each v € V. G is a complete graph if E contains every pair of elements

more »

... of V. A graph H = (V 1 , E ! ) is a partial graph of G=(V,E) if V Çv and E ! Ç E. H is a restriction of G if H is a partial graph of G in which V ! = V. Let S = { e ,. . . , e } be a subset of E such that