Abstract
In [5], Salinas asked the following question: If T = (Ti,.. , Tn) consists of commuting hyponormal operators, is it true that (1) 5(T A) = d(A, a, (T)) and (2) r, (T) = I IDT II? He proved that, for a doubly commuting n-tuple of quasinormal operators, (2) was true and (1) was true for A = 0. In this paper we shall show that (2) holds for a doubly commuting n-tuple of hyponormal operators and give an example of a subnormal operator which does not satisfy (1).
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