Department of Mathematics, National Central University, Chung-Li 32001, Taiwan ; Department of Applied Mathematics, National Chiao Tung University, Hsinchu 300, Taiwan
Abstract:
It is shown that if is a compact operator on a Hilbert space with its numerical range contained in the closed unit disc and with intersecting the unit circle at infinitely many points, then is equal to . This is an infinite-dimensional analogue of a result of Anderson for finite matrices.