The Spectra of Nonnegative Integer Matrices via Formal Power Series

An old problem in matrix theory is to determine the n-tuples of complex numbers which can occur as the spectrum of a matrix with nonnegative entries (see [BP94, Chapter 4] or [Min88, Chapter VII]). Authors have studied the case where the ntuple is comprised of real numbers [Bor95, Cia68, Fri78, Kel71, Per53, Sal72, Sou83, Sul49], the case where the matrices under consideration are symmetric [Fie74, JLL96], and the general problem [Joh81, LM99, LL79, Rea94, Rea96, Wuw97]. Various necessary conditions and sufficient conditions have been provided, but a complete characterization is known for real n-tuples only for n ≤ 4 [Kel71, Sul49] and for complex n-tuples only for n ≤ 3 [LL79]. Motivated by symbolic dynamics, Boyle and Handelman refocused attention on the nonzero part of the spectrum by making the following “Spectral Conjecture” [BH91, BH93] (see also [Boy93, §8] and [LM95, Chapter 11]). Below, a matrix A is primitive if all entries of A are nonnegative and for some n, all entries of A are strictly positive. Also,