The Commutant of Multiplication by z on the Closure of Rational Functions in Lt(μ)L^t(\mu)

For a compact set $K\subset \mathbb C,$ a finite positive Borel measure $\mu$ on $K,$ and 1 \le t < \i, let $\text{Rat}(K)$ be the set of rational functions with poles off $K$ and let $R^t(K, \mu)$ be the closure of $\text{Rat}(K)$ in $L^t(\mu).$ For a bounded Borel subset $\mathcal D\subset \mathbb C,$ let \area_{\mathcal D} denote the area (Lebesgue) measure restricted to $\mathcal D$ and let H^\i (\mathcal D) be the weak-star closed sub-algebra of L^\i(\area_{\mathcal D}) spanned by $f,$ bounded and analytic on $\mathbb C\setminus E_f$ for some compact subset $E_f \subset \mathbb C\setminus \mathcal D.$ We show that if $R^t(K, \mu)$ contains no non-trivial direct $L^t$ summands, then there exists a Borel subset $\mathcal R \subset K$ whose closure contains the support of $\mu$ and there exists an isometric isomorphism and a weak-star homeomorphism $\rho$ from $R^t(K, \mu) \cap L^\infty(\mu)$ onto $H^\infty(\mathcal R)$ such that $\rho(r) = r$ for all $r\in\text{Rat}(K).$ Consequently, we obtain some structural decomposition theorems for \rtkmu.Comment: arXiv admin note: text overlap with arXiv:2212.1081

Invertibility in Weak-Star Closed Algebras of Analytic Functions

For $K\subset \mathbb C$ a compact subset and $\mu$ a positive finite Bore1 measure supported on $K,$ let $R^\infty (K,\mu)$ be the weak-star closure in $L^\infty (\mu)$ of rational functions with poles off $K.$ We show that if $R^\infty (K,\mu)$ has no non-trivial $L^\infty$ summands and $f\in R^\infty (K,\mu),$ then $f$ is invertible in $R^\infty (K,\mu)$ if and only if Chaumat's map for $K$ and $\mu$ applied to $f$ is bounded away from zero on the envelope with respect to $K$ and $\mu.$ The result proves the conjecture $\diamond$ posed by J. Dudziak in 1984.Comment: arXiv admin note: text overlap with arXiv:2212.1081