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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 ΘH(D)\Theta \in H^\infty(\mathbb D) and [g][g] in the quotient algebra H/ΘHH^\infty/ \Theta H^\infty,

its quotient norm is 

[g]:=inf{g+Θh,hH}\|[g]\|:= \inf \left\{ \|g+\Theta h\|_\infty, h \in H^\infty \right\}. We show that 

when gg is normalized so that [g]=1\|[g]\|=1, the quotient norm of its inverse can be made 

arbitrarily close to 11 by imposing g(z)1δ|g(z)|\ge 1- \delta when Θ(z)=0\Theta(z)=0, with \delta>0 small enough, 

(call this property SIP)

 if and only if the function Θ\Theta satisfies the following growth property:

</p><p>limt1inf{Θ(z):zD,ρ(z,Θ1{0})t}=1,</p><p></p><p>\lim_{t\to 1} \inf\left\{ |\Theta(z)|: z \in \mathbb D, \rho(z, \Theta^{-1} \{0\} ) \ge t \right\} =1,</p><p>

where ρ\rho is the usual pseudohyperbolic distance in the disc, ρ(z,w):=zw1zwˉ\rho(z,w):= \left| \frac{z-w}{1-z\bar w}\right|.


We prove that an inner function is SIP if and only if for any \eps&gt;0, the set \{ z: 0&lt; |\Theta (z) | &lt; 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.