A Contribution to Best Approximation in the l2 - Norm
This paper characterizes the commutant of certain multiplication operators on Hilbert spaces of analytic functions. Let be the operator of multiplication by z on the underlying Hilbert space. We give sufficient conditions for an operator essentially commuting with A and commuting with for some n>1 to be the operator of multiplication by an analytic symbol. This extends a result of Shields and Wallen.
Here we consider when the difference of two composition operators is compact on the weighted Dirichlet spaces . Specifically we study differences of composition operators on the Dirichlet space and S 2, the space of analytic functions whose first derivative is in H 2, and then use Calderón’s complex interpolation to extend the results to the general weighted Dirichlet spaces. As a corollary we consider composition operators induced by linear fractional self-maps of the disk.