Published online by Cambridge University Press: 20 November 2018
In 1954 A.J. Hoffman and O. Taussky [1] showed that if A is an n-square complex matrix with eigenvalues λ = (λ1, …, λn ) and P is a permutation matrix for which αA + βA* has eigenvalues for some αβ ≠ 0 then A is normal. Here
is the conjugate vector of λ. As a companion result they also proved that if the eigenvalues of AA* are
, i = 1, …, n then A is normal.