Symmetric matrix representations of truncated Toeplitz operators on finite dimensional spaces

TL;DR

This paper characterizes when 3-by-3 symmetric matrices can represent truncated Toeplitz operators on finite-dimensional spaces, providing necessary and sufficient conditions and addressing an open conjecture in the field.

Contribution

It offers a complete characterization of matrix representations of truncated Toeplitz operators in finite dimensions, including conditions for symmetry and conjugation invariance, and resolves an open conjecture.

Findings

Necessary and sufficient conditions for 3x3 symmetric matrices to represent truncated Toeplitz operators.

Counterexample showing not all unitary equivalences arise from modified Clark basis.

Polynomial system characterizes valid matrix representations with real solutions.

Abstract

In this paper, we study matrix representations of truncated Toeplitz operators with respect to orthonormal bases which are invariant under a canonical conjugation map. In particular, we determine necessary and sufficient conditions for when a 3-by-3 symmetric matrix is the matrix representation of a truncated Toeplitz operator with respect to a given conjugation invariant orthonormal basis. We specialise our result to the case when the conjugation invariant orthonormal basis is a modified Clark basis. As a corollary to this specialisation, we answer a previously stated open conjecture in the negative, and show that not every unitary equivalence between a complex symmetric matrix and a truncated Toeplitz operator arises from a modified Clark basis representation. Finally, we show that a given 3-by-3 symmetric matrix is the matrix representation of a truncated Toeplitz operator with respect to a conjugation invariant orthonormal basis if and only if a specified system of polynomial equations is satisfied with a real solution.