Abstract
A homogeneous set is a set of vertices H of a graph G=(V,E) such that each vertex of V\H is either adjacent to all vertices of H or none of them. A graph Gs=(V,Es) is a sandwich for a pair of graph Gt=(V,Et) and G=(V,E) if Es is included in E and contains Et. In a recent paper of Tang et al, a polynomial algorithm is described for computing all the possible homogeneous sets for the sandwich graphs of Gt and G. In this paper, we invalidate this algorithm by proving there are exponentially many such sets. We give a correct characterization of a homogeneous set of a sandwich graph.