On The Doubly Stochastic Realization Of Spectra

Abstract

An $n$-list $\lambda:=\left(r; \lambda_2, \ldots, \lambda_n\right)$ of complex numbers with $r>0,$ is said to be realizable if $\lambda$ is the spectrum of $n\times n$ nonnegative matrix $A$ and in this case $A$ is said to be a nonnegative realization of $\lambda$. If, in addition, each row and column sum of $A$ equals $r$, then $\lambda$ is said to be doubly stochastically realizable and in such case $A$ is said to be a doubly stochastic realization for $\lambda$. In 1997, Guo proved that if $\left(\lambda_2,\ldots, \lambda_n\right)$ is any list of complex numbers which is closed under complex conjugation then there exists a least real number $\lambda_0$ with $\max\limits_{2\leq j\leq n}|\lambda_j|\leq\lambda_0\leq 2n\max\limits_{2\leq j\leq n}|\lambda_j|$ such that the list of complex numbers $\{ \lambda_1,\lambda_2,...,\lambda_n\}$ is realizable if and only if $\lambda_1\geq \lambda_0$. In 2020, Julio and Soto showed that the upper bound may be reduced to $(n-1)\max\limits_{2\leq j\leq n}|\lambda_j|$ in the case when at least one of the $\lambda_i$ is real. In this paper, we first describe an algorithm for passing from a nonnegative realization to a doubly stochastic realization. As applications, we give a new sufficient condition for a stochastic matrix $A$ to be cospectral to a doubly stochastic matrix $B$ and in this case $B$ is shown to be the unique closest doubly stochastic matrix to $A$ with respect to the Frobenius norm. Then, our next results slightly improve the upper bound for the nonnegative realization presented by Julio and Soto and in the case when none of the $\lambda_i$ is real, we also give an improvement of Guo's bound. Then, for doubly stochastic realizations, we obtain an upper bound that improves Guo's bound as well. Finally, for certain particular cases, we give a further improvement of our last bound for doubly stochastic realization.