The Laplacian matrix of weighted threshold graphs

Threshold graphs are generated from one node by repeatedly adding a node that links to all existing nodes or adding a node without links. In the weighted threshold graph, we add a new node in step i, which is linked to all existing nodes by a link of weight wi . In this work, we consider the set AN that contains all Laplacian matrices of weighted threshold graphs of order N. We show that AN forms a commutative algebra. Using this, we find a common basis of eigenvectors for the matrices in AN . It follows that the eigenvalues of each matrix in AN can be represented as a linear transformation of the link weights. In addition, we prove that, if there are just three or fewer different weights, two weighted threshold graphs with the same Laplacian spectrum must be isomorphic.