Id\'eaux ferm\'es de certaines alg\`ebres de Beurling et application aux op\'erateurs \`a spectre d\'enombrable

Abstract

We denote by $\bbt$ the unit circle and by $\bbd$ the unit disc of $\bbc$. Let $s$ be a non-negative real and $\omega$ a weight such that $\omega(n) = (1+n)^{s} \quad (n \geq 0)$ and such that the sequence $\dsp \Big(\frac{\omega(-n)}{(1+n)^{s}} \Big)_{n \geq 0}$ is non-decreasing. We define the Banach algebra $$ A_{\omega}(\bbt) = \Big\{f \in \calc(\bbt) : \big\| f \big\|_{\omega} = \sum_{n = -\infty}^{+\infty} | \hat{f}(n) | \omega(n) < +\infty \Big\}, $$ If $I$ is a closed ideal of $A_{\omega}(\bbt)$, we set $h^{0}(I) = \Big\{z \in \bbt : f(z) = 0 \quad (f \in I) \Big\}$. We describe here all closed ideals $I$ of $A_{\omega}(\bbt)$ such that $h^{0}(I)$ is at most countable. A similar result is obtained for closed ideals of the algebra $A_{s}^{+}(\bbt) = \Big\{f \in A_{\omega}(\bbt) : \hat{f}(n) = 0 \quad (n<0) \Big\}$ without inner factor. Then, we use this description to establish a link between operators with countable spectrum and interpolating sets for $\textrm{{\LARGE $a$}}^{\infty}$, the space of infinitely differentiable functions in the closed unit disc $\bar{\bbd}$ and holomorphic in $\bbd$.