Model Theory for Operators Related to Square Roots of Normal Operators

Abstract

In this paper we prove that a square root of a cyclic normal operator is unitarily equivalent to a block multiplication operator on a vector-valued Lebesgue space with a $2$--normal symbol. In addition, we show that a cyclic operator which admits a cyclic $2$--normal extension is unitarily equivalent to a block multiplication operator on a corresponding vector-valued Hardy space with $2$--normal symbol. We consider the existence of bounded point evaluations in the vector-valued Hardy space setting; as an application, we prove that an operator admitting a cyclic $2$--normal extension has nontrivial invariant subspaces. We also study uniqueness for minimal $n$--normal extensions of operators that have cyclic $2$--normal extensions.