Simply invariant subspaces of H2 of some multiply connected regions
(1996) In Integral Equations and Operator Theory 24(2). p.127155 Abstract
 The authors are concerned with the invariant subspaces of $H^2(\Omega)$, where $\Omega$ is a bounded, finitely connected, planar domain with an analytic boundary. A subspace of $H^2(\Omega)$ is said to be invariant if it is invariant under multiplication by $z$, and fully invariant if it is invariant under multiplication by all rational functions with poles off the closure of $\Omega$.
If $\Omega$ is simply connected, every invariant subspace is fully invariant; these subspaces are described by the famous theorem of A. Beurling from 1949. For fully invariant subspaces in the multiply connected case, several authors showed in the 1960s that an analogue of Beurling's theorem holds. The first description of invariant... (More)  The authors are concerned with the invariant subspaces of $H^2(\Omega)$, where $\Omega$ is a bounded, finitely connected, planar domain with an analytic boundary. A subspace of $H^2(\Omega)$ is said to be invariant if it is invariant under multiplication by $z$, and fully invariant if it is invariant under multiplication by all rational functions with poles off the closure of $\Omega$.
If $\Omega$ is simply connected, every invariant subspace is fully invariant; these subspaces are described by the famous theorem of A. Beurling from 1949. For fully invariant subspaces in the multiply connected case, several authors showed in the 1960s that an analogue of Beurling's theorem holds. The first description of invariant subspaces that are not fully invariant was obtained by H. L. Royden [Pacific J. Math. 134 (1988), no. 1, 151172; MR0953505 (90a:46056)] and D. Hitt [Pacific J. Math. 134 (1988), no. 1, 101120; MR0953502 (90a:46059)]; they treated the case where $\Omega$ is an annulus. Their work was extended to the general finitely connected case by D. V. Yakubovich [Zap. Nauchn. Sem. Leningrad. Otdel. Mat. Inst. Steklov. (LOMI) 178 (1989), Issled. Lineĭn. Oper. Teorii Funktsiĭ. 18, 166183, 186187; MR1037771 (91c:47061)]. The description of an invariant subspace that is not fully invariant involves a property of pseudocontinuation across one or more of the interior boundary components of $\Omega$.
In the present paper the authors present an alternative approach to the results of Royden, Hitt and Yakubovich. The main new ingredient is a norm equality, suggested by recent work in Bergman spaces, which the authors use in handling the case of an annulus. In addition, the authors study the operator of multiplication by $z$ on an invariant subspace, determining the spectrum and approximate point spectrum, and obtaining an upper bound for its cyclic multiplicity.
As the authors note, it would be desirable to sharpen several points in their description of invariant subspaces. They obtain partial results in this direction. In the erratum, the authors replace a flawed lemma (Lemma 2.3) from the original paper. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/record/1467236
 author
 Aleman, Alexandru ^{LU} and Richter, Stefan
 publishing date
 1996
 type
 Contribution to journal
 publication status
 published
 subject
 in
 Integral Equations and Operator Theory
 volume
 24
 issue
 2
 pages
 127  155
 publisher
 Springer
 external identifiers

 scopus:0030074899
 ISSN
 14208989
 DOI
 10.1007/BF01193457
 language
 English
 LU publication?
 no
 id
 50a202a14399488fafb43f65974d970d (old id 1467236)
 date added to LUP
 20090916 13:22:15
 date last changed
 20180107 05:21:45
@article{50a202a14399488fafb43f65974d970d, abstract = {The authors are concerned with the invariant subspaces of $H^2(\Omega)$, where $\Omega$ is a bounded, finitely connected, planar domain with an analytic boundary. A subspace of $H^2(\Omega)$ is said to be invariant if it is invariant under multiplication by $z$, and fully invariant if it is invariant under multiplication by all rational functions with poles off the closure of $\Omega$. <br/><br> <br/><br> If $\Omega$ is simply connected, every invariant subspace is fully invariant; these subspaces are described by the famous theorem of A. Beurling from 1949. For fully invariant subspaces in the multiply connected case, several authors showed in the 1960s that an analogue of Beurling's theorem holds. The first description of invariant subspaces that are not fully invariant was obtained by H. L. Royden [Pacific J. Math. 134 (1988), no. 1, 151172; MR0953505 (90a:46056)] and D. Hitt [Pacific J. Math. 134 (1988), no. 1, 101120; MR0953502 (90a:46059)]; they treated the case where $\Omega$ is an annulus. Their work was extended to the general finitely connected case by D. V. Yakubovich [Zap. Nauchn. Sem. Leningrad. Otdel. Mat. Inst. Steklov. (LOMI) 178 (1989), Issled. Lineĭn. Oper. Teorii Funktsiĭ. 18, 166183, 186187; MR1037771 (91c:47061)]. The description of an invariant subspace that is not fully invariant involves a property of pseudocontinuation across one or more of the interior boundary components of $\Omega$. <br/><br> <br/><br> In the present paper the authors present an alternative approach to the results of Royden, Hitt and Yakubovich. The main new ingredient is a norm equality, suggested by recent work in Bergman spaces, which the authors use in handling the case of an annulus. In addition, the authors study the operator of multiplication by $z$ on an invariant subspace, determining the spectrum and approximate point spectrum, and obtaining an upper bound for its cyclic multiplicity. <br/><br> <br/><br> As the authors note, it would be desirable to sharpen several points in their description of invariant subspaces. They obtain partial results in this direction. In the erratum, the authors replace a flawed lemma (Lemma 2.3) from the original paper.}, author = {Aleman, Alexandru and Richter, Stefan}, issn = {14208989}, language = {eng}, number = {2}, pages = {127155}, publisher = {Springer}, series = {Integral Equations and Operator Theory}, title = {Simply invariant subspaces of H2 of some multiply connected regions}, url = {http://dx.doi.org/10.1007/BF01193457}, volume = {24}, year = {1996}, }