Francisco-Enrique Velasco

Normal matrices

Let $A$ be a square complex matrix and $z$ a complex number. The distance, with respect to the spectral norm, from $A$ to the set of matrices which have $z$ as an eigenvalue is less than or equal to the distance from $z$ to the spectrum of $A$. If these two distances are equal for a sufficiently large finite set of numbers $z$ which are not in the spectrum of $A$, then the matrix $A$ is normal.