Minimality of $T_z^*$-Invariant Subspaces of $H^2(\mathbb D^2)$

Abstract

In this paper, we establish necessary conditions for the minimality of infinite-dimensional $T_z^*$-invariant subspaces of $H^2(\mathbb D^2)$. More precisely, we prove that if $M\subset H^2(\mathbb D^2)$ is an infinite-dimensional, minimal $T_z^*$-invariant subspace, then, for every $n\in\mathbb N_0$, either $\overline{P_n(M)}=H^2(\mathbb D)$ or $\overline{P_n(M)}^\perp =\phi_{a,\theta}H^2(\mathbb D)$ where $\phi_{a,\theta}$ is an atomic singular inner function and $P_n$ denotes the orthogonal projection of $H^2(\mathbb D^2)$ onto $H^2(z)w^n$. As $T_z^*$ is a universal operator on $H^2(\mathbb D^2)$, this result reduces the Invariant Subspace Problem to the analysis of these two alternatives for infinite-dimensional minimal $T_z^*$-invariant subspaces.