Overlap number of graphs

An overlap representation of a graph G assigns sets to vertices so that vertices are adjacent if and only if their assigned sets intersect with neither containing the other. The overlap number φ(G) (introduced by Rosgen) is the minimum size of the union of the sets in such a representation. We prove the following: (1) An optimal overlap representation of a tree can be produced in linear time, and its size is the number of vertices in the largest subtree in which the neighbor of any leaf has degree 2. (2) If δ(G) ≥ 2 and G 6= K3, then φ(G) ≤ |E(G)| − 1, with equality when G is connected and triangle-free and has no star-cutset. (3) If G is an n-vertex plane graph with n ≥ 5, then φ(G) ≤ 2n− 5, with equality when every face has length 4 and there is no star-cutset. (4) If G is an n-vertex graph with n ≥ 14, then φ(G) ≤ n2/4 − n/2 − 1, with equality for even n when G arises from Kn/2,n/2 by deleting a perfect matching.