Interlacing for weighted graphs using the normalized Laplacian

Abstract

The problem of relating the eigenvalues of the normalized Laplacian for a weighted graph G and G - H, for H a subgraph of G is considered. It is shown that these eigenvalues interlace and that the tightness of the interlacing is dependent on the number of nonisolated vertices of H. Weak coverings of a weighted graph are also defined and interlacing results for the normalized Laplacian for such a covering are given. In addition there is a discussion about interlacing for the Laplacian of directed graphs.