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# Wydawnictwa / Czasopisma IMPAN / Studia Mathematica / Wszystkie zeszyty

## $L^1$ factorizations, moment problems and invariant subspaces

### Tom 167 / 2005

Studia Mathematica 167 (2005), 183-194 MSC: 47A15, 47A57, 30D55. DOI: 10.4064/sm167-2-5

#### Streszczenie

For an absolutely continuous contraction $T$ on a Hilbert space ${{\mathcal H}}$, it is shown that the factorization of various classes of $L^1$ functions $f$ by vectors $x$ and $y$ in ${{\mathcal H}}$, in the sense that $\langle T^nx,y\rangle = \widehat f(-n)$ for $n \ge 0$, implies the existence of invariant subspaces for $T$, or in some cases for rational functions of $T$. One of the main tools employed is the operator-valued Poisson kernel. Finally, a link is established between $L^1$ factorizations and the moment sequences studied in the Atzmon–Godefroy method, from which further results on invariant subspaces are derived.

#### Autorzy

• Isabelle ChalendarInstitut Girard Desargues, UFR de Mathématiques
Université Claude Bernard Lyon 1
69622 Villeurbanne Cedex, France
e-mail
• Jonathan R. PartingtonSchool of Mathematics
University of Leeds
Leeds LS2 9JT, U.K.
e-mail
• Rachael C. SmithSchool of Mathematics
University of Leeds
Leeds LS2 9JT, U.K.
e-mail

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