We know (e.g. [Godsil, Royle: Algebraic Graph Theory, Lemma 13.2.3]Lemma 13.2.3]) that any cofactor of the Laplacian matrix of a graph is constant, and is equal to the number of spanning trees of the graph. How do the cofactors change if I just add a diagonal matrix to the Laplacian matrix?
Any help would be greatly appreciated.