Projections in the Algebra generated by an n-Potent Operator

Abstract

This paper investigates the projection operators that lie in the algebra generated by powers of an $n$-potent operator $T$ on a complex Banach space, where $T^n=T$. We give a complete description of all projections in the algebra $\mathrm{comb}(T)=\text{span}\{T,T^2,\dots ,T^{n-1}\}$, and prove that each such projection is uniquely determined by, and in bijection with, a subset of the nonzero spectrum of $T$. As a consequence, the family of projections in $\mathrm{comb}(T)$ forms a Boolean algebra isomorphic to the power set of $\ddot{I}ƒ(T)\setminus \{0\}$. We also establish a spectral decomposition for $n$-potent operators in terms of their Riesz projections and derive explicit formulas for the associated Riesz projections using resolvent expansions. We give an illustration of the theory for $5$-potent operators, which highlights the algebraic and spectral structure of finite-order operators on Banach spaces.