We apply eigenvalue interlacing techniques for obtaining lower and upper bounds for the sums of Laplacian eigenvalues of graphs, and characterize equality. This leads to generalizations of, and variations on theorems by Grone, and Grone & Merris. As a consequence we obtain inequalities involving bounds for some well-known parameters of a graph, such as edge-connectivity, and the isoperimetric number.
Eigenvalue interlacingThroughout this paper, G = (V, E) is a finite simple graph with n = |V | vertices. Theorem 1.1. Let A be a real symmetric n × n matrix with eigenvalues λ 1 ≥ · · · ≥ λ n . For some m < n, let S be a real n × m matrix with orthonormal columns, S ⊤ S = I, and consider the matrix B = S ⊤ AS, with eigenvalues µ 1 ≥ · · · ≥ µ m . Then, (a) the eigenvalues of B interlace those of A, that is,(b) if the interlacing is tight, that is, for some 0 ≤ k ≤ m, λ i = µ i , i = 1, . . . , k, and µ i = λ n−m+i , i = k + 1, . . . , m, then SB = AS.