\Bibitem{Sta74} \by J.~G.~Stampfli \paper Hyponormal operators \jour Matematika \yr 1974 \vol 18 \issue 1 \pages 90--94 \mathnet{http://mi.mathnet.ru/mat708} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=0149282} \zmath{https://zbmath.org/?q=an:0129.08701} \transl \jour Pacific J. Math. \yr 1962 \vol 12 \pages 1453--1458