Abstract

It is said that a list Λ = {λ1, …, λn} of complex numbers is realizable, if it is the spectrum of a nonnegative matrix A. It is said that Λ is universally realizable if it is realizable for each possible Jordan canonical form allowed by Λ. This work does not contain new results. As its title says, its goal is to show and emphasize the relevance and importance of certain results, by Brauer and Rado, in the study of nonnegative inverse spectral problems. It is shown that virtually all known results, which give sufficient conditions for Λ to be realizable or universally realizable, can be obtained from results by Brauer and Rado. Moreover, from these results, a realizing matrix may always be constructed.