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Compressions of resolvents and maximal radius of regularity
Authors:C Badea  M Mbekhta
Institution:URA 751 au CNRS & UFR de Mathématiques, Université de Lille I, F--59655 Villeneuve d'Ascq, France ; URA 751 au CNRS & UFR de Mathématiques, Université de Lille I, F--59655 Villeneuve d'Ascq, France
Abstract:Suppose that $\lambda - T$ is left invertible in $L(H)$ for all $\lambda \in \Omega$, where $\Omega$ is an open subset of the complex plane. Then an operator-valued function $L(\lambda)$ is a left resolvent of $T$ in $\Omega$ if and only if $T$ has an extension $\tilde{T}$, the resolvent of which is a dilation of $L(\lambda)$ of a particular form. Generalized resolvents exist on every open set $U$, with $\overline{U}$ included in the regular domain of $T$. This implies a formula for the maximal radius of regularity of $T$ in terms of the spectral radius of its generalized inverses. A solution to an open problem raised by
J. Zemánek is obtained.

Keywords:One-sided resolvents  Hilbert space operators  spectral radius  dilations and compressions
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