Author

C. Badea

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Preprint Oct 2026

The Gleason--Kahane--Zelazko theorem: a gentle introduction

After some personal recollections of Jean-Pierre Kahane, this paper gives a gentle introduction to the Gleason-Kahane-Zelazko theorem. The theorem states that a normalised complex-linear functional on a unital complex Banach algebra is multiplicative whenever it does not vanish on invertible elements. Neither continuit...

C. Badea · 0 citations

Preprint Sep 2026

Iterates of Ritt operators close to the identity

We study bounded operators whose prescribed powers remain uniformly close to the identity, and resolvent conditions sampled along sequences of powers. The displacement bound $\sup_{n\ge1}\|I-T^n\|\le q<2$ forces a finite peripheral spectrum consisting of odd-order roots of unity and makes an explicitly determined odd p...

C. Badea · 0 citations