Some classes of graphs that are not PCGs

A graph G = (V, E) is a pairwise compatibility graph (PCG) if there exists an edge-weighted tree T and two non-negative real numbers dmin and dmax, dmin ≤ dmax, such that each node u ∈ V is uniquely associated to a leaf of T and there is an edge (u, v) ∈ E if and only if dmin ≤ dT (u, v) ≤ dmax, where dT (u, v) is the sum of the weights of the edges on the unique path PT (u, v) from u to v in T . Understanding which graph classes lie inside and which ones outside the PCG class is an important issue. In this paper we propose a new proof technique that allows us to show that some interesting classes of graphs have empty intersection with PCG. As an example, we use this technique to show that wheels and graphs obtained as strong product between a cycle and P2 are not PCGs. keywords: Phylogenetic Tree Reconstruction Problem, Pairwise Compatibility Graphs (PCGs), PCG Recognition Problem, Wheel.