Mustafayev, H.S.Hüseynov, F.B.2025-05-102025-05-1020142662-203310.15352/bjma/13966400472-s2.0-84898762600https://doi.org/10.15352/bjma/1396640047https://hdl.handle.net/20.500.14720/4850Let T be a bounded, linear operator on a complex, separable, infinite dimensional Hilbert space H. We assume that T is an essentially isometric (resp. normal) operator, that is, IH-T*T (resp. TT*-T*T) is compact. For the compactness of S from the commutant of T, some necessary and sufficient conditions are found on S. Some related problems are also discussed.eninfo:eu-repo/semantics/closedAccess(Essential) SpectrumCompact OperatorEssentially Unitary (Normal) OperatorFunctional CalculusArticle82Q2Q2115WOS:000336224300001