A spectral excess theorem for normal digraphs

Abstract

It is known that every distance-regular digraph is connected and normal. An interesting question is: when is a given connected normal digraph distance-regular? Motivated by this question first we give some characterizations of weakly distanceregular digraphs. Specially we show that whether a given connected digraph to be weakly distance-regular only depends on the equality for two invariants. Then we show that a connected normal digraph Γ with d + 1 distinct eigenvalues is distance-regular if and only if the simple excess (the ratio of the square of mean of the numbers of shortest paths between vertices at distance d to the mean of the numbers of vertices at distance d from every vertex, which is zero if d is greater than the diameter) is equal to the spectral excess (a number which can be computed from the spectrum of Γ). In fact, this result is a new variation (a simple variation) of the spectral excess theorem due to Fiol and Garigga for connected normal digraphs. Using these results we derive another variation (a weighted variation) of the spectral excess theorem for connected normal digraphs. Distance regularity of a digraph (also a graph) is in general not determined by its spectrum. For application of the simple variation we show that distance regularity of a connected normal digraph Γ (with d + 1 distinct eigenvalues) is determined by its spectrum and the invariant δ d (the mean of the numbers of vertices at distance d from every vertex). Finally as an application of the weighted variation we show that every connected normal digraph Γ with d + 1 distinct eigenvalues and diameter D is either a bipartite digraph, or a generalized odd graph or it has odd-girth at most min{2d − 1, 2D + 1}, generalizing a result of van Dam and Haemers and also a recent result of Lee and Weng.