Homothetic polygons and beyond: Maximal cliques in intersection graphs

Abstract

We study the structure and the maximum number of maximal cliques in classes of intersection graphs of convex sets in the plane. It is known that convex-set intersection graphs, and also straight-line-segment intersection graphs may have exponentially many maximal cliques. On the other hand, in intersection graphs of homothetic triangles, the maximum number of maximal cliques is polynomial in the number of vertices. We extend the latter result by showing that for every convex polygon P with sides parallel to k directions, every n-vertex graph which is an intersection graph of homothetic copies of P contains at most nk inclusion-wise maximal cliques. We actually prove this result for a more general class of graphs, the so-called kDIR-CONV, which are intersection graphs of convex polygons whose sides are parallel to some fixed k directions. Moreover, we provide lower bounds on the maximum number of maximal cliques and generalize the upper bound to intersection graphs of higher-dimensional convex polytopes in Euclidean space. Finally, we discuss the algorithmic consequences of the polynomial bound on the number of maximal cliques.