English

Two problems on submodules of $H^2(\mathbb{D}^n)$

Functional Analysis 2024-06-14 v1 Complex Variables

Abstract

Given any shift-invariant closed subspace S\mathcal{S} (aka submodule) of the Hardy space over the unit polydisc H2(Dn)H^2(\mathbb{D}^n) (where n2n \geq 2), let Rzj:=MzjSR_{z_j}:=M_{z_j}|_{\mathcal{S}}, and Ezj:=PSevzjE_{z_j}:=P_{\mathcal{S}}\circ ev_{z_j}, for each j{1,,n}j \in \{1,\ldots,n\}. Here, evzjev_{z_j} is the operator evaluating at 00 in the zjz_j-th variable. In this article, we prove that given any subset Λ{1,,n}\Lambda \subseteq \{1,\ldots,n\}, there exists a collection of one-variable inner functions {ϕλ(zλ)}λΛ\{\phi_\lambda (z_\lambda)\}_{\lambda \in \Lambda} on Dn\mathbb{D}^n, such that S=λΛϕλ(zλ)H2(Dn), \mathcal{S} = \sum_{\lambda \in \Lambda} \phi_\lambda (z_\lambda)H^2(\mathbb{D}^n), if and only if the conditions (ISEzkEzk)(ISRzkRzk)=0 (I_{\mathcal{S}}-E_{z_k}E_{z_k}^*)(I_{\mathcal{S}}-R_{z_k}R_{z_k}^*)=0 for all k{1,,n}Λk \in \{1,\dots,n\} \setminus \Lambda, and (ISEziEzi)(ISRziRzi)(ISEzjEzj)(ISRzjRzj)=0(I_{\mathcal{S}}-E_{z_{i}}E_{z_{i}}^*)(I_{\mathcal{S}}-R_{z_{i}}R_{z_{i}}^*)(I_{\mathcal{S}}-E_{z_{j}}E_{z_{j}}^*)(I_{\mathcal{S}}-R_{z_{j}}R_{z_{j}}^*)=0 for all distinct i,jΛi,j \in \Lambda are satisfied. Following this, we study R.G. Douglas's question on the commutativity of orthogonal projections onto the corresponding quotient modules.

Keywords

Cite

@article{arxiv.2406.09245,
  title  = {Two problems on submodules of $H^2(\mathbb{D}^n)$},
  author = {Ramlal Debnath and Srijan Sarkar},
  journal= {arXiv preprint arXiv:2406.09245},
  year   = {2024}
}

Comments

Preliminary version. Comments are welcome!

R2 v1 2026-06-28T17:04:45.843Z