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#### Keywords

Interlacing inequalities, Inverse eigenvalue problem, Symmetric matrix

#### Abstract

Let G be a simple undirected graph on n vertices and let S(G) be the class of real symmetric n by n matrices whose nonzero off-diagonal entries correspond to the edges of G. Given 2n - 1 real numbers \lambda_1\geq \mu_1 \geq \lambda_2 \geq \mu_2 \geq \cdots \geq \lambda_{n-1} \geq \mu_{n-1} \geq \lambda_{n-1}, and a vertex v of G, the question is addressed of whether or not there exists A in S(G) with eigenvalues \lambda_1, \ldots, \lambda_ n such that A(v) has eigenvalues \mu_1, \ldots, \mu_{n-1}, where A(v) denotes the matrix with vth row and column deleted. A complete solution can be given for the path on n vertices with v a pendant vertex and also for the star on n vertices with v the dominating vertex. The main result is a complete solution to this "\lambda, \mu" problem for all connected graphs on 4 vertices.

#### Recommended Citation

Barrett, Wayne; Nelson, Curtis G.; Sinkovic, John Henry; and Yang, Tianyi.
(2014),
"The combinatorial inverse eigenvalue problem II: all cases for small graphs",
*Electronic Journal of Linear Algebra*,
Volume 27.

DOI: https://doi.org/10.13001/1081-3810.1938

*Abstract*