On Mixed Almost Moore Graphs of Diameter Two

Mixed almost Moore graphs appear in the context of the Degree/Diameter problem as a class of extremal mixed graphs, in the sense that their order is one less than the Moore bound for mixed graphs. The problem of their existence has been considered before for directed graphs and undirected ones, but not for the mixed case, which is a kind of generalization. In this paper we give some necessary conditions for the existence of mixed almost Moore graphs of diameter two derived from the factorization in Q[x] of their characteristic polynomial. In this context, we deal with the irreducibility of Φi ( x2 + x − (r − 1) ) , where Φi(x) denotes the i-th cyclotomic polynomial.