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Transactions of the American Mathematical Society

ISSN 1088-6850(online) ISSN 0002-9947(print)



The commutant of an analytic Toeplitz operator

Author: Carl C. Cowen
Journal: Trans. Amer. Math. Soc. 239 (1978), 1-31
MSC: Primary 47B35; Secondary 30A78
MathSciNet review: 0482347
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Abstract: For a function f in $ {H^\infty }$ of the unit disk, the operator on $ {H^2}$ of multiplication by f will be denoted by $ {T_f}$ and its commutant by $ \{ {T_f}\} '$. For a finite Blaschke product B, a representation of an operator in $ {\{ {T_B}\}'}$ as a function on the Riemann surface of $ {B^{ - 1}} \circ B$ motivates work on more general functions. A theorem is proved which gives conditions on a family $ \mathcal{F}$ of $ {H^\infty }$ functions which imply that there is a function h such that $ \{ {T_h}\} ' = { \cap _{f \in \mathcal{F}}}\{ {T_f}\} '$. As a special case of this theorem, we find that if the inner factor of $ f - f(c)$ is a finite Blaschke product for some c in the disk, then there is a finite Blaschke product B with $ \{ {T_f}\} ' = \{ {T_B}\} '$. Necessary and sufficient conditions are given for an operator to commute with $ {T_f}$ when f is a covering map (in the sense of Riemann surfaces). If f and g are in $ {H^\infty }$ and $ f = h \circ g$, then $ \{ {T_f}\} ' \supset \{ {T_g}\} '$. This paper introduces a class of functions, the $ {H^2}$-ancestral functions, for which the converse is true. If f and g are $ {H^2}$-ancestral functions, then $ \{ {T_f}\} ' \ne \{ {T_g}\} '$ unless $ f = h \circ g$ where h is univalent. It is shown that inner functions and covering maps are $ {H^2}$-ancestral functions, although these do not exhaust the class. Two theorems are proved, each giving conditions on a function f which imply that $ {T_f}$ does not commute with nonzero compact operators. It follows from one of these results that if f is an $ {H^2}$-ancestral function, then $ {T_f}$ does not commute with any nonzero compact operators.

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Keywords: Toeplitz operator, commutant, $ {H^\infty },{H^2}$, analytic function, inner function, universal covering map
Article copyright: © Copyright 1978 American Mathematical Society