Definition. Let G and H be simple graphs with the same vertex set V. We say that G and H are complements if for every u, v V with u v, there is an edge with ends u, v in exactly one of the graphs G or H. We use G to denote a complement of the graph G. 7. For every positive integer k construct a graph Gk with 4k vertices with the property that Gk is isomorphic to Gk.