Contents

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Necessary Spectral Conditions for Coloring Hypergraphs

Franklin H. J. Kenter1
1University of California, San Diego; La Jolla, CA 92093

Abstract

Hoffman proved that for a simple graph G, the chromatic number χ(G) obeys χ(G)1λ1λn, where λ1 and λn are the maximal and minimal eigenvalues of the adjacency matrix of G, respectively. Lovász later showed that χ(G)1λ1λn for any (perhaps negatively) weighted adjacency matrix.

In this paper, we give a probabilistic proof of Lovász’s theorem, then extend the technique to derive generalizations of Hoffman’s theorem when allowed a certain proportion of edge-conflicts. Using this result, we show that if a 3-uniform hypergraph is 2-colorable, then d¯32λmin where d¯ is the average degree and λmin is the minimal eigenvalue of the underlying graph. We generalize this further for k-uniform hypergraphs, for the cases k=4 and 5, by considering several variants of the underlying graph.