Carlos M. Saiago

The Parter-Wiener Theorem: Refinement and Generalization

An important theorem about the existence of principal submatrices of a Hermitian matrix whose graph is a tree, in which the multiplicity of an eigenvalue increases, was largely developed in separate papers by Parter and Wiener. Here, the prior work is fully stated, then generalized with a self-contained proof. The more complete result is then used to better understand the eigenvalue possibilities of reducible principal submatrices of Hermitian tridiagonal matrices. Sets of vertices, for which the multiplicity increases, are also studied.

On the relative position of multiple eigenvalues in the spectrum of an Hermitian matrix with a given graph

For Hermitian matrices, whose graph is a given tree, the relationships among vertex degrees, multiple eigenvalues and the relative position of the underlying eigenvalue in the ordered spectrum are discussed in detail. In the process, certain aspects of special vertices, whose removal results in an increase in multiplicity are investigated.

Inverse eigenvalue problems and lists of multiplicities of eigenvalues for matrices whose graph is a tree: the case of generalized stars and double generalized stars

We characterize the possible lists of ordered multiplicities among matrices whose graph is a generalized star (a tree in which at most one vertex has degree greater than 2) or a double generalized star. Here, the inverse eigenvalue problem (IEP) for symmetric matrices whose graph is a generalized star is settled. The answer is consistent with a conjecture that determination of the possible ordered multiplicities is equivalent to the IEP for a given tree. Moreover, a key spectral feature of the IEP in the case of generalized stars is shown to characterize them among trees.

ESTIMATION OF THE MAXIMUM MULTIPLICITY OF AN EIGENVALUE IN TERMS OF THE VERTEX DEGREES OF THE GRAPH OF A MATRIX

The maximum multiplicity among eigenvaluesof matriceswith a given graph cannot generally be expressed in terms of the degrees of the vertices (even when the graph is a tree). Given are best possible lower and upper bounds, and characterization of the cases of equality in these bounds. A by-product is a sequential algorithm to calculate the exact maximum multiplicity by simple counting.

The change in eigenvalue multiplicity associated with perturbation of a diagonal entry

We investigate the relation between perturbing the i-th diagonal entry of A ∈ ℳ n (𝔽) and extracting the principal submatrix A(i) from A with respect to the possible changes in multiplicity of a given eigenvalue. A complete description is given and used to both generalize and improve prior work about Hermitian matrices whose graph is a given tree.

Diameter minimal trees

Using the method of seeds and branch duplication, it is shown that for every tree of diameter , there is an Hermitian matrix with as few as distinct eigenvalues (a known lower bound). For diameter 7, some trees require 8 distinct eigenvalues, but no more; the seeds for which 7 and 8 are the worst case are classified. For trees of diameter , it is shown that, in general, the minimum number of distinct eigenvalues is bounded by a function of . Many trees of high diameter permit as few of distinct eigenvalues as the diameter and a conjecture is made that all linear trees are of this type. Several other specific, related observations are made.

The trees for which maximum multiplicity implies the simplicity of other eigenvalues

Among those real symmetric matrices whose graph is a given tree T, the maximum multiplicity is known to be the path cover number of T. An explicit characterization is given for those trees for which whenever the maximum multiplicity is attained, all other multiplicities are 1.

Classification of vertices and edges with respect to the geometric multiplicity of an eigenvalue in a matrix, with a given graph, over a field

Abstract We are interested in the geometric multiplicity of an identified eigenvalue of a matrix A over a general field with a given graph G for its off-diagonal entries. By the classification of a vertex (edge) of G, we refer to the change in the geometric multiplicity of when this vertex (edge) is removed from G to leave a principal submatrix (modification) of A. Such classification in the case of Hermitian matrices and trees has been strategic in the problem of determining the possible lists of multiplicities for the eigenvalues among matrices with the given graph. Here, our view of, and qualification of, the classification in the general setting provides a tool that makes some past arguments more transparent and provides new insight in both the classical and general setting. Some applications are given to general downer mechanisms for the recognition of Parter vertices and to the stability of geometric multiplicity under perturbation of a diagonal entry.