Inertia and spectral symmetry of eccentricity matrices of some clique trees

Abstract

The eccentricity matrix $\mathcal{E}(G)$ of a connected graph $G$ is obtained from the distance matrix of $G$ by leaving unchanged the largest nonzero entries in each row and each column, and replacing the remaining ones with zeros. In this paper, we consider the set $\mathcal{C}\mathcal{T}$ of clique trees whose blocks have at most two cut-vertices \textcolor{blue}{of the clique tree}. After proving the irreducibility of the eccentricity matrix of a clique tree in $\mathcal{C}\mathcal{T}$ and finding its inertia indices, we show that every graph in $\mathcal{C}\mathcal{T}$ with more than $4$ vertices and odd diameter has two positive and two negative $\mathcal{E}$-eigenvalues. Positive $\mathcal{E}$-eigenvalues and negative $\mathcal{E}$-eigenvalues turn out to be equal in number even for graphs in $\mathcal{C}\mathcal{T}$ with even diameter; that shared cardinality also counts the \textcolor{blue}{`diametrally distinguished'} vertices. Finally, we prove that the spectrum of the eccentricity matrix of a clique tree $G$ in $\mathcal{C}\mathcal{T}$ is symmetric with respect to the origin if and only if $G$ has an odd diameter and exactly two adjacent central vertices.

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