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A graph for which the inertia bound is not tight


The inertia bound gives an upper bound on the independence number of a graph by considering the inertia of matrices corresponding to the graph. The bound is known to be tight for graphs on 10 or fewer vertices as well as for all perfect graphs. It is natural to question whether the bound is always tight. We show that the bound is not tight for the Paley graph on 17 vertices as well as its induced subgraph on 16 vertices.

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Correspondence to John Sinkovic.

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Sinkovic, J. A graph for which the inertia bound is not tight. J Algebr Comb 47, 39–50 (2018).

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  • Inertia bound
  • Cvetković bound
  • Independence number
  • Weight matrix

Mathematics Subject Classification

  • 05C50
  • 05C69
  • 15A42