Invariant subspaces of the direct sum of forward and backward shifts on vector-valued Hardy spaces

TL;DR

This paper characterizes invariant subspaces of the direct sum of forward and backward shift operators on vector-valued Hardy spaces, linking them to bilateral shift invariant subspaces and expressing them via Toeplitz and Hankel operators.

Contribution

It establishes a one-to-one correspondence between these invariant subspaces and those of bilateral shifts, extending previous results to higher-dimensional vector spaces.

Findings

Invariant subspaces correspond to kernels or ranges of mixed Toeplitz and Hankel operators.

Provides new proofs and extends results to higher-dimensional vector spaces.

Expresses invariant subspaces in terms of partial isometry-valued symbols.

Abstract

Let $S_{E}$ be the shift operator on vector-valued Hardy space $H_{E}^{2}.$ Beurling-Lax-Halmos Theorem identifies the invariant subspaces of $S_{E}$ and hence also the invariant subspaces of the backward shift $S_{E}^{\ast}.$ In this paper, we study the invariant subspaces of $S_{E}\oplus S_{F}^{\ast}.$ We establish a one-to-one correspondence between the invariant subspaces of $S_{E}\oplus S_{F}^{\ast}$ and a class of invariant subspaces of bilateral shift $B_{E}\oplus B_{F}$ which were described by Helson and Lowdenslager. As applications, we express invariant subspaces of $S_{E}\oplus S_{F}^{\ast}$ as kernels or ranges of mixed Toeplitz operators and Hankel operators with partial isometry-valued symbols. Our approach greatly extends and gives different proofs of the results of C\^{a}mara and Ross, and Timotin where the case with one dimensional $E$ and $F$ was considered.