Maximal ideal space of some Banach algebras of Dirichlet series

Let $\mathscr{H}^\infty$ be the set of all Dirichlet series $f=\sum_{n=1}^\infty \frac{a_n}{n^s}$ (where $a_n\in \mathbb{C}$ for each $n$) that converge at each $s\in \mathbb{C}_+:=ż\in\mathbb{C}:\text{Re}(z)>0\}$, such that $\|f\|_{\infty}:=\sup_{s\in \mC_+}|f(s)|<\infty$. Let $\mathscr{B}\subset \mathscr{H}^\infty$ be a Banach algebra containing the Dirichlet polynomials (Dirichlet series with finitely many nonzero terms) with a norm $\|\cdot\|_{\mathscr{B}}$ such that the inclusion $\mathscr{B} \subset \mathscr{H}^\infty$ is continuous.% (i.e., there exists a $C>0$ such that $\|f\|_{\infty} \le C\|f\|_{\mathscr{B}}$ for all $f\in \mathscr{B}$). For $m\in \mathbb{N}=\{1,2,3,\ldots\}$, let $\partial^{{{-m}}}\mathscrB$ denote the Banach algebra consisting of all $f\in \mathscr{B}$ such that $f',\ldots, f^{{{(m)}}}\in \mathscr{B}$, with pointwise operations and the norm $\|f\|_{\partial^{-m}\mathscr{B}}=\sum_{\ell=0}^m \frac{1}{\ell!}\|f^{(\ell)}\|_{\mathscr{B}}$. Assuming that the Wiener $1/f$ property holds for $\mathscr{B}$ (that is, $\inf_{s\in \mathbb{C}_+} |f(s)|>0$ implies $1/f\in \mathscr{B}$), it is shown that for all $m\in \mathbb{N}$, the maximal ideal space $\text{M}(\partial^{{{-m}}}\mathscr{B})$ of $\partial^{{{-m}}}\mathscr{B}$ is homeomorphic to $\overline{\mathbb{D}}^{\mathbb{N}}$, where $\overline{\mathbb{D}}=\{z\in \mathbb{C}:|z|\le 1\}$. Examples of such Banach algebras are $\mathscr{H}^\infty$, the subalgebra $\mathscr{A}_u$ of $\mathscr{H}^\infty$ consisting of uniformly continuous functions in $\mathbb{C}_+$, and the Wiener algebra $\mathscr{W}$ of Dirichlet series with $\|f\|_{{{\mathscrW}}}:={{\sum_{n=1}^\infty}} |a_n|<\infty$. Some consequences (existence of logarithms, projective freeness, infinite Bass stable rank) are given as applications.

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Publication Details

Journal
Functiones et Approximatio Commentarii Mathematici
Published
2025-01-01
DOI
https://doi.org/10.7169/facm/250317-12-10
Primary Topic
Advanced Banach Space Theory
Type
article
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article

Maximal ideal space of some Banach algebras of Dirichlet series

Amol Sasane
Functiones et Approximatio Commentarii Mathematici
Advanced Banach Space Theory
article

Maximal ideal space of some Banach algebras of Dirichlet series

Amol Sasane
article en

Abstract

Let $\mathscr{H}^\infty$ be the set of all Dirichlet series $f=\sum_{n=1}^\infty \frac{a_n}{n^s}$ (where $a_n\in \mathbb{C}$ for each $n$) that converge at each $s\in \mathbb{C}_+:=ż\in\mathbb{C}:\text{Re}(z)>0\}$, such that $\|f\|_{\infty}:=\sup_{s\in \mC_+}|f(s)|<\infty$. Let $\mathscr{B}\subset \mathscr{H}^\infty$ be a Banach algebra containing the Dirichlet polynomials (Dirichlet series with finitely many nonzero terms) with a norm $\|\cdot\|_{\mathscr{B}}$ such that the inclusion $\mathscr{B} \subset \mathscr{H}^\infty$ is continuous.% (i.e., there exists a $C>0$ such that $\|f\|_{\infty} \le C\|f\|_{\mathscr{B}}$ for all $f\in \mathscr{B}$). For $m\in \mathbb{N}=\{1,2,3,\ldots\}$, let $\partial^{{{-m}}}\mathscrB$ denote the Banach algebra consisting of all $f\in \mathscr{B}$ such that $f',\ldots, f^{{{(m)}}}\in \mathscr{B}$, with pointwise operations and the norm $\|f\|_{\partial^{-m}\mathscr{B}}=\sum_{\ell=0}^m \frac{1}{\ell!}\|f^{(\ell)}\|_{\mathscr{B}}$. Assuming that the Wiener $1/f$ property holds for $\mathscr{B}$ (that is, $\inf_{s\in \mathbb{C}_+} |f(s)|>0$ implies $1/f\in \mathscr{B}$), it is shown that for all $m\in \mathbb{N}$, the maximal ideal space $\text{M}(\partial^{{{-m}}}\mathscr{B})$ of $\partial^{{{-m}}}\mathscr{B}$ is homeomorphic to $\overline{\mathbb{D}}^{\mathbb{N}}$, where $\overline{\mathbb{D}}=\{z\in \mathbb{C}:|z|\le 1\}$. Examples of such Banach algebras are $\mathscr{H}^\infty$, the subalgebra $\mathscr{A}_u$ of $\mathscr{H}^\infty$ consisting of uniformly continuous functions in $\mathbb{C}_+$, and the Wiener algebra $\mathscr{W}$ of Dirichlet series with $\|f\|_{{{\mathscrW}}}:={{\sum_{n=1}^\infty}} |a_n|<\infty$. Some consequences (existence of logarithms, projective freeness, infinite Bass stable rank) are given as applications.

Functiones et Approximatio Commentarii MathematiciVol. -1(-1)
Openalex Percentile: Top 55%
Advanced Banach Space Theory
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Maximal ideal space of some Banach algebras of Dirichlet series — Amol Sasane · Functiones et Approximatio Commentarii Mathematici (2025) | TGRS Research Map | TGRS