Geodetic spectra of graphs

Geodetic numbers of graphs and digraphs have been investigated in the literature recently. The main purpose of this paper is to study the geodetic spectrum of a graph. For any two vertices u and v in an oriented graph D, a u-v geodesic is a shortest directed path from u to v. Let I (u, v) denote the set of all vertices lying on a u-v geodesic. For a vertex subset A, let I (A) denote the union of all I (u, v) for u, V is an element of A. The geodetic number g (D) of an oriented graph D is the minimum cardinality of a set A with I (A) = V(D). The (strong) geodetic spectrum of a graph G is the set of geodetic numbers of all (strongly connected) orientations of G. In this paper, we determine geodetic spectra and strong geodetic spectra of several classes of graphs. A conjecture and two problems given by Chartrand and Zhang are dealt with. (C) 2003 Published by Elsevier Ltd.