Nearly invariant subspaces and weighted dual truncated Toeplitz operators

Abstract

Let $\mathcal{M}=h{\mathcal{K}}_u$ be a nearly $S^{\ast }$-invariant subspace of $H^2$, where ${\mathcal{K}}_u=H^2\ominus u{H}^2$ and $h$ is the extremal multiplier. For $\vp\in L^\infty(\T)$, we study the compression \[ D_\vp^\mathcal{M} = \restr{P_{\mathcal{M}^\perp}M_\vp}{\mathcal{M}^\perp}, \] called a weighted dual truncated Toeplitz operator. When $h\equiv 1$, this reduces to the classical dual truncated Toeplitz operator. Using the Hartmann--Ross projection formula, we prove \[ \|D_\vp^\mathcal{M}\|=\|\vp\|_\infty, \] characterize compactness, and show that the natural multiplication map by $h$ identifies the weighted and classical theories precisely when $h$ is inner. We also establish complex symmetry and obtain block matrix, defect, and semi-commutator identities via weighted truncated Hankel operators. As a main algebraic consequence, we prove \[ D_\vp^\mathcal{M} D_ψ^\mathcal{M}=0 \quad\Longleftrightarrow\quad \vp=0\ \text{or}\ ψ=0 \quad\text{a.e. on }\T. \] Finally, we derive a a rank-at-most-two correction formula and a finite-rank displacement identity for the generalized dual shift $D_z^{\mathcal{M}}$.

28 pages