Towards a functional calculus for subnormal tuples: the minimal normal extension

Conway, John B.

doi:10.1090/S0002-9947-1991-1005077-X

Towards a functional calculus for subnormal tuples: the minimal normal extension
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Trans. Amer. Math. Soc. 326 (1991), 543-567 Request permission

Abstract:

In this paper the study of a functional calculus for subnormal $n$-tuples is initiated and the minimal normal extension problem for this functional calculus is explored. This problem is shown to be equivalent to a mean approximation problem in several complex variables which is solved. An analogous uniform approximation problem is also explored. In addition these general results are applied together with The Area and the The Coarea Formula from Geometric Measure Theory to operators on Bergman spaces and to the tensor product of two subnormal operators. The minimal normal extension of the tensor product of the Bergman shift with itself is completely determined.

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Invariant subspaces for joint subnormal systems