On clique convergent graphs

Abstract

A graph G is convergent when there is some finite integer n≥0, such that the n-th iterated clique graph Kn (G) has only one vertex. The smallest such n is the index of G. The Helly defect of a convergent graph is the smallest h such that Kʰ(G) is clique Helly, that is, its maximal cliques satisfy the Helly property. Bandelt and Prisner proved that the Helly defect of a chordal graph is at most one and asked whether there is a graph whose Helly defect exceeds the difference of its index and diameter by more than one. In the present paper an affirmative anwer to the question is given. For any arbitrary finite integer n, it is exhibited a graph, the Helly defect of which exceeds by n the difference of its index and diameter.