Nevanlinna--Pick norms: towards a scattered--Cantor dichotomy for spectra of commutative Banach algebras

Abstract

We introduce Nevanlinna--Pick norms associated with finite families of characters in a commutative semisimple Banach algebra and study the class $NP_\infty$, where all such norms are minimal. Our main result is a topological rigidity theorem: if $A\in NP_\infty$ and $K\subset\Delta(A)$ is compact scattered, then the restriction algebra $NP(A,K)$ is isometrically $C(K)$. Consequently, if $\Delta(A)$ is compact scattered, then $A\in NP_\infty$ precisely when $A$ is isometrically $C(\Delta(A))$ under the Gelfand transform. This applies, in particular, to ordinal intervals and one-point compactifications of generalized Mrowka spaces. Conversely, every compact Hausdorff space containing a Cantor subset occurs as the spectrum of a commutative unital Banach algebra $A\in NP_\infty$ with $A\ne C(\Delta(A))$. We also discuss uniform algebras: examples with all points peak points belong to $NP_\infty$, and $NP_\infty$ is equivalent to all Gleason parts being singletons.