For a bounded linear operator T in a Banach space the Ritt resolvent condition (|λ| > 1) can be extended (changing the constant C) to any sector |arg(λ - 1)| ≤ π - δ, . This implies the power boundedness of the operator T. A key result is that the spectrum σ(T) is contained in a special convex closed domain. A generalized Ritt condition leads to a similar localization result and then to a theorem on invariant subspaces.
@article{bwmeta1.element.bwnjournal-article-smv134i2p153bwm, author = {Yu. Lyubich}, title = {Spectral localization, power boundedness and invariant subspaces under Ritt's type condition}, journal = {Studia Mathematica}, volume = {133}, year = {1999}, pages = {153-167}, zbl = {0945.47005}, language = {en}, url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-smv134i2p153bwm} }
Lyubich, Yu. Spectral localization, power boundedness and invariant subspaces under Ritt's type condition. Studia Mathematica, Tome 133 (1999) pp. 153-167. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-smv134i2p153bwm/
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