Let $\Gamma$ be a regular (connected) graph with $n$ vertices and $d+1$ distinct eigenvalues. As a main result, it is shown that $\Gamma$ is an $r$-antipodal distance-regular graph if and only if the distance graph $\Gamma_d$ is constituted by disjoint coies of the complete graph $K_r$, with $r$ satisfying an expression in terms of $n$ and the distinct eigenvalues.
CitationFiol Mora, Miquel Àngel. An eigenvalue characterization of antipodal distance-regular graphs. The electronic journal of combinatorics, 1997, 4, 1, #R30
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