Cyclicity in the Drury-Arveson space and other weighted Besov spaces

Abstract

Let $\mathcal{H}$ be a space of analytic functions on the unit ball $\mathbb B_d$ in $\mathbb C^d$ with multiplier algebra $\mathrm{Mult}(\mathcal{H})$. A function $f\in \mathcal{H}$ is called cyclic if the set $[f]$, the closure of $\{\varphi f:\varphi \in \mathrm{Mult}(\mathcal{H})\}$, equals $\mathcal{H}$. For multipliers we also consider a weakened form of the cyclicity concept. Namely for $n\in \mathbb N_0$ we consider the classes $$\mathcal{C}_n(\mathcal{H})=\{\varphi \in \mathrm{Mult}(\mathcal H):\varphi\ne 0, [\varphi^n]=[\varphi^{n+1}]\}.$$ Many of our results hold for $N$:th order radially weighted Besov spaces on $\mathbb B_d$, but we describe our results only for the Drury-Arveson space $H^2_d$ here. Letting $\mathbb C_{stable}[z]$ denote the stable polynomials for $\mathbb B_d$, i.e. the $d$-variable complex polynomials without zeros in $\mathbb B_d $, we show that \begin{align*} &\text{ if }d \text{ is odd, then } \mathbb C_{stable}[z]\subseteq \mathcal C_{\frac{d-1}{2}}(H^2_d), \text{ and }\\ &\text{ if }d \text{ is even, then } \mathbb C_{stable}[z]\subseteq \mathcal C_{\frac{d}{2}-1}(H^2_d).\end{align*} For $d=2$ and $d=4$ these inclusions are the best possible, but in general we can only show that if $0\le n\le \frac{d}{4}-1$, then $\mathbb C_{stable}[z]\nsubseteq \mathcal C_n(H^2_d)$. For functions other than polynomials we show that if $f,g\in H^2_d$ such that $f/g\in H^\infty$ and $f$ is cyclic, then $g$ is cyclic. We use this to prove that if $f,g\in H^2_d$ extend to be analytic in a neighborhood of $\overline{\mathbb B_d }$, have no zeros in $\mathbb B_d $, and their zero sets coincide on the boundary, then $f$ is cyclic if and only if $g$ is cyclic. Furthermore, if for $f\in H^2_d\cap C(\overline{\mathbb B_d })$ the set $Z(f)\cap \partial \mathbb B_d$ embeds a cube of real dimension $\ge 3$, then $f$ is not cyclic in the Drury-Arveson space.