On Suffridge polynomials

Abstract

We consider some known and some new properties of the family of polynomials introduced by Ted Suffridge in 1969. We begin by giving a brief overview of their extremal properties in classic and more recent work. We also give a compact form for Suffridge polynomials which matches a general pattern discovered by Brandt. Our approach allows us to find the coefficients which Brandt's result was not giving explicitly. This new presentation provides us the tools to obtain an estimate of the rate of approximation of the generalized Koebe functions by univalent polynomials. Furthermore, we consider the presentation of Suffridge polynomials in Robertson's form and find the suiting Robertson measure. This suggests a new way to approximate step functions by continuous monotonic ones. We then study the lack of robustness of the univalency of these polynomials and suggest a new family of polynomials for which we conjecture the univalency of a subclass. Namely, we prove the quite surprising fact that by extending the family by letting the discrete argument in the polynomial coefficients become continuous one does not increase the set of univalent polynomials. Only the initial polynomials remain univalent. In this new one parameter family generalizing the Suffridge polynomials, it is remarkable that the Suffridge polynomials are already extremal as they correspond to the choice of the parameter set to 1; moreover the complex Fej\'er polynomials correspond the choice of the parameter set to 0, and the choice of the parameter set to -1 corresponds the polynomials $z+(z^N/N)$. Remarkably, computer simulations seem to clearly indicate that the image of the unit disc under these new polynomial mapping is a simply-connected region bounded by a simple curve. This justifies the conjectural univalency of these polynomials for the whole range of the parameters.