Nodal domain theorems and bipartite subgraphs

Abstract

The Discrete Nodal Domain Theorem states that an eigenfunction of the k-th largest
eigenvalue of a generalized graph Laplacian has at most k (weak) nodal domains. The number of
strong nodal domains is shown not to exceed the size of a maximal induced bipartite subgraph and
that this bound is sharp for generalized graph Laplacians. Similarly, the number of weak nodal
domains is bounded by the size of a maximal bipartite minor.