Oscillation of graph eigenfunctions

Abstract

An oscillation formula is established for the $k$-th eigenvector (assumed to be simple and with non-zero entries) of a weighted graph operator. The formula directly attributes the number of sign changes exceeding $k-1$ to the cycles in the graph, by identifying it as the Morse index of a weighted cycle intersection form introduced in the text. Two proofs are provided for the main result. Additionally, it is related to the nodal--magnetic theorem of Berkolaiko and Colin de Verdi\`ere and to a similar identity of Bronski, DeVille and Ferguson obtained for the linearization of coupled oscillator network equations around a known solution.