Symmetric matrix representations of truncated Toeplitz operators on finite dimensional spaces

Ryan O'Loughlin

Published 2022-02-03Version 1

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.