Petersen graph
= Petersen graph
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The <graph> whose vertices are two-element subsets of a five-element set, with two vertices adjacent when the subsets are disjoint. It has ten vertices, degree three, zero common neighbours for adjacent pairs and one for nonadjacent pairs. Its <adjacency matrix of a graph> satisfies $A^2+A-2I=J$ and has <eigenvalues> $3,1,-2$ of multiplicities $1,5,4$.