On the Recognition of Bipolarizable and P_4-simplicial Graphs

The classes of Raspail (also known as Bipolarizable) and P_4-simplicial graphs were introduced by Hoàng and Reed who showed that both classes are perfectly orderable and admit polynomial-time recognition algorithms HR1. In this paper, we consider the recognition problem on these classes of graphs and present algorithms that solve it in O(n m) time. In particular, we prove properties and show that we can produce bipolarizable and P_4-simplicial orderings on the vertices of the input graph G, if such orderings exist, working only on P_3s that participate in a P_4 of G. The proposed recognition algorithms are simple, use simple data structures and both require O(n + m) space. Additionally, we show how our recognition algorithms can be augmented to provide certificates, whenever they decide that G is not bipolarizable or P_4-simplicial; the augmentation takes O(n + m) time and space. Finally, we include a diagram on class inclusions and the currently best recognition time complexities for a number of perfectly orderable classes of graphs

On the Recognition of P4-Indifferent Graphs

A simple graph is P_4-indifferent if it admits a total order b>c>d. P_4-indifferent graphs generalize indifferent graphs and are perfectly orderable. Recently, Hoang, Maffray and Noy gave a characterization of P_4-indifferent graphs in terms of forbidden induced subgraphs. We clarify their proof and describe a linear time algorithm to recognize P_4-indifferent graphs. When the input is a P_4-indifferent graph, then the algorithm computes an order < as above