A counter-example to a conjecture of Friedland

Abstract

In 1982, S. Friedland proved that a bounded linear operator A on a Hilbert space is normal if and only if (αI + A + A*)² ≧ AA* − A*A ≧ −(αI + A + A*)² for all real α. And he conjectured the inequality (αI + A + A*)² ≧ AA* − A*A for all real α will imply that A*A − AA* ≧ 0, i.e., A is hyponormal. But his conjecture is incorrect. In this note I’ll give a counter-example for his conjecture.