Order spectrum of the Ces\`aro operator in Banach lattice sequence spaces

Abstract

The discrete Ces\`aro operator $ C $ acts continuously in various classical Banach sequence spaces within $ \mathbb{C}^{\mathbb{N}}.$ For the coordinatewise order, many such sequence spaces $ X $ are also complex Banach lattices (eg. $c_0, \ell^p $ for $ 1 < p \leq \infty , $ and $ ces (p)$ for $ p \in \{ 0 \} \cup ( 1, \infty )).$ In such Banach lattice sequence spaces, $ C $ is always a positive operator. Hence, its order spectrum is well defined within the Banach algebra of all regular operators on $ X .$ The purpose of this note is to show, for every $ X $ belonging to the above list of Banach lattice sequence spaces, that the order spectrum $ \sigma_{\rm o} (C)$ of $ C $ coincides with its usual spectrum $ \sigma ( C)$ when $ C $ is considered as a continuous linear operator on the Banach space $ X .$