Incidence matrices of projective planes and of some regular bipartite graphs of girth 6 with few vertices

Abstract

Let q be a prime power and r=0,1...,q−3. Using the Latin squares obtained by multiplying each entry of the addition table of the Galois field of order q by an element distinct from zero, we obtain the incidence matrices of projective planes and the incidence matrices of (q−r)-regular bipartite graphs of girth 6 and $q^2$−rq−1 vertices in each partite set. Moreover, in this work two Latin squares of order q−1 with entries belonging to {0,1,..., q}, not necessarily the same, are defined to be quasi row-disjoint if and only if the cartesian product of any two rows contains at most one pair (χ,χ) with χ≠0. Using these quasi row-disjoint Latin squares we find (q−1)-regular bipartite graphs of girth 6 with $q^2$−q−2 vertices in each partite set. Some of these graphs have the smallest number of vertices known so far among the regular graphs with girth 6.