Séminaire d'Analyse
Sharp Invertibility in Quotient Algebras of $H^\infty$
Pascal Thomas
( Toulouse )Salle de conférences
le 06 juin 2024 à 14:00
Given an inner function and in the quotient algebra ,
its quotient norm is
. We show that
when is normalized so that , the quotient norm of its inverse can be made
arbitrarily close to by imposing when , with \delta>0 small enough,
(call this property SIP)
if and only if the function satisfies the following growth property:
where is the usual pseudohyperbolic distance in the disc, .
We prove that an inner function is SIP if and only if for any \eps>0, the set \{ z: 0< |\Theta (z) | < 1-\eps\}
cannot contain hyperbolic disks of arbitrarily large radius.
Thin Blaschke products provide an example of such functions. Some SIP Blaschke products fail to be interpolating
(and thus aren't thin), while there exist Blaschke products which are interpolating and fail to be SIP.
We also study the functions which can be divisors of SIP inner functions.