Orthogonal Polynomials, a Szegő--Verblunsky Theorem and Baxter's Theorem on the Quaternionic Sphere

We introduce a theory of orthogonal polynomials on the unit sphere of the quaternions based on the notion of a $q$-positive measure (which originated in a work of Alpay, Colombo, the second author and Sabadini). The results we extend to this setting include the Szegő recurrences, the Zeros Theorem for orthogonal polynomials, the Szegő–Verblunsky theorem, and Baxter’s theorem; to obtain these results, we utilise the Verblunsky coefficients (or Schur parameters) of Alpay, Colombo and Sabadini and a number of established results in the matricial setting. Our approach also requires matrix-valued analogues of Schur’s recurrences for the coefficients of a Schur function and of Verblunsky’s formula for the moments of a measure, which appear to be new.