Asymptotic limits, Banach limits, and Ces\`aro means

Abstract

Every new inner product in a Hilbert space is obtained from the original one by means of a unique positive operator$.$ The first part of the paper is a survey on applications of such a technique, including a characterization of similarity to isometries$.$ The second part focuses on Banach limits for dealing with power bounded operators. It is shown that if a power bounded operator for which the sequence of shifted Ces\`aro means converges (at least in the weak topology) uniformly in the shift parameter, then it has a Ces\`aro asymptotic limit coinciding with its $\varphi$-asymptotic limit for all Banach limits $\varphi$.