Constructing New Realisable Lists from Old in the NIEP

Abstract

Given a list of complex numbers \sigma:=(\lambda_1,\lambda_2,...,\lambda_m), we say that {\sigma} is realisable if {\sigma} is the spectrum of some (entrywise) nonnegative matrix. The Nonnegative Inverse Eigenvalue Problem (or NIEP) is the problem of categorising all realisable lists. Given a realisable list (\rho,\lambda_2,\lambda_3,...,\lambda_m), where {\rho} is the Perron eigenvalue and \lambda_2 is real, we find families of lists (\mu_1,\mu_2,...,\mu_n), for which (\mu_1,\mu_2,...,\mu_n,\lambda_3,\lambda_4,...,\lambda_m) is realisable. In addition, given a realisable list (\rho,\alpha+i\beta,\alpha-i\beta,\lambda_4,\lambda_5,...,\lambda_m), where {\rho} is the Perron eigenvalue and {\alpha} and {\beta} are real, we find families of lists (\mu_1,\mu_2,\mu_3,\mu_4), for which (\mu_1,\mu_2,\mu_3,\mu_4,\lambda_4,\lambda_5,...,\lambda_m) is realisable.