Nontangential limits in Pt(µ)spaces and the index of invariant subgroups
(2009) In Annals of Mathematics 169(2). p.449490 Abstract
 Abstract
Let μ be a finite positive
measure on the closed disk D¯
in the complex plane, let 1 ≤ t < ∞,
and let Pt(μ)
denote the closure of the analytic polynomials in
Lt(μ). We suppose
that D
is the set of analytic bounded point evaluations for
Pt(μ), and
that Pt(μ)
contains no nontrivial characteristic functions. It is then known that the restriction of
μ to
∂D must be of the form
hdz. We prove that every
function f ∈ Pt(μ) has nontangential
limits at hdzalmost
every point of ∂D,
and the... (More)  Abstract
Let μ be a finite positive
measure on the closed disk D¯
in the complex plane, let 1 ≤ t < ∞,
and let Pt(μ)
denote the closure of the analytic polynomials in
Lt(μ). We suppose
that D
is the set of analytic bounded point evaluations for
Pt(μ), and
that Pt(μ)
contains no nontrivial characteristic functions. It is then known that the restriction of
μ to
∂D must be of the form
hdz. We prove that every
function f ∈ Pt(μ) has nontangential
limits at hdzalmost
every point of ∂D,
and the resulting boundary function agrees with
f as an
element of Lt(hdz).
Our proof combines methods from James E. Thomson’s proof of the existence of bounded point
evaluations for Pt(μ)
whenever Pt(μ)≠Lt(μ)
with Xavier Tolsa’s remarkable recent results on analytic capacity. These methods allow
us to refine Thomson’s results somewhat. In fact, for a general compactly supported
measure ν
in the complex plane we are able to describe locations of bounded point evaluations
for Pt(ν) in
terms of the Cauchy transform of an annihilating measure.
As a consequence of our result we answer in the affirmative a conjecture of Conway and Yang. We
show that for 1 < t < ∞ dim
ℳ∕zℳ = 1 for every nonzero
invariant subspace ℳ
of Pt(μ) if and
only if h≠0.
We also investigate the boundary behaviour of the functions in
Pt(μ) near the
points z ∈ ∂D
where h(z) = 0. In
particular, for 1 < t < ∞
we show that there are interpolating sequences for
Pt(μ)
that accumulate nontangentially almost everywhere on
{z : h(z) = 0}. (Less)
Please use this url to cite or link to this publication:
https://lup.lub.lu.se/record/1467113
 author
 Aleman, Alexandru ^{LU} ; Richter, Stefan and Sundberg, Carl
 organization
 publishing date
 2009
 type
 Contribution to journal
 publication status
 published
 subject
 in
 Annals of Mathematics
 volume
 169
 issue
 2
 pages
 449  490
 publisher
 Annals of Mathematics
 external identifiers

 scopus:71649104937
 ISSN
 0003486X
 language
 English
 LU publication?
 yes
 id
 bbe3a04b16404e05a55e9333d23f70a8 (old id 1467113)
 date added to LUP
 20160401 12:12:28
 date last changed
 20210901 03:53:43
@article{bbe3a04b16404e05a55e9333d23f70a8, abstract = {Abstract<br/><br> <br/><br> <br/><br> <br/><br> <br/><br> <br/><br> <br/><br> <br/><br> Let μ be a finite positive<br/><br> measure on the closed disk D¯<br/><br> in the complex plane, let 1 ≤ t < ∞,<br/><br> and let Pt(μ)<br/><br> denote the closure of the analytic polynomials in<br/><br> Lt(μ). We suppose<br/><br> that D<br/><br> is the set of analytic bounded point evaluations for<br/><br> Pt(μ), and<br/><br> that Pt(μ)<br/><br> contains no nontrivial characteristic functions. It is then known that the restriction of<br/><br> μ to<br/><br> ∂D must be of the form<br/><br> hdz. We prove that every<br/><br> function f ∈ Pt(μ) has nontangential<br/><br> limits at hdzalmost<br/><br> every point of ∂D,<br/><br> and the resulting boundary function agrees with<br/><br> f as an<br/><br> element of Lt(hdz).<br/><br> <br/><br> Our proof combines methods from James E. Thomson’s proof of the existence of bounded point<br/><br> evaluations for Pt(μ)<br/><br> whenever Pt(μ)≠Lt(μ)<br/><br> with Xavier Tolsa’s remarkable recent results on analytic capacity. These methods allow<br/><br> us to refine Thomson’s results somewhat. In fact, for a general compactly supported<br/><br> measure ν<br/><br> in the complex plane we are able to describe locations of bounded point evaluations<br/><br> for Pt(ν) in<br/><br> terms of the Cauchy transform of an annihilating measure.<br/><br> <br/><br> As a consequence of our result we answer in the affirmative a conjecture of Conway and Yang. We<br/><br> show that for 1 < t < ∞ dim<br/><br> ℳ∕zℳ = 1 for every nonzero<br/><br> invariant subspace ℳ<br/><br> of Pt(μ) if and<br/><br> only if h≠0.<br/><br> <br/><br> We also investigate the boundary behaviour of the functions in<br/><br> Pt(μ) near the<br/><br> points z ∈ ∂D<br/><br> where h(z) = 0. In<br/><br> particular, for 1 < t < ∞<br/><br> we show that there are interpolating sequences for<br/><br> Pt(μ)<br/><br> that accumulate nontangentially almost everywhere on<br/><br> {z : h(z) = 0}.}, author = {Aleman, Alexandru and Richter, Stefan and Sundberg, Carl}, issn = {0003486X}, language = {eng}, number = {2}, pages = {449490}, publisher = {Annals of Mathematics}, series = {Annals of Mathematics}, title = {Nontangential limits in Pt(µ)spaces and the index of invariant subgroups}, volume = {169}, year = {2009}, }