# Christoffel formula for kernel polynomials on the unit circle

## TL;DR

This paper derives a formula linking Christoffel-Darboux kernels for measures on the unit circle when the measure is modified by a polynomial factor, and explores how this affects associated recurrence parameters and specific measure examples.

## Contribution

It provides a determinantal formula connecting kernels of original and modified measures and analyzes the impact on recurrence coefficients for orthogonal polynomials.

## Key findings

- Derived a determinantal formula for kernel transformation under polynomial measure modification
- Established relations between recurrence parameters of original and modified measures
- Applied results to specific measures like Geronimus weight and hypergeometric functions

## Abstract

Given a nontrivial positive measure $\mu$ on the unit circle, the associated Christoffel-Darboux kernels are $K_n(z, w;\mu) = \sum_{k=0}^{n}\overline{\varphi_{k}(w;\mu)}\,\varphi_{k}(z;\mu)$, $n \geq 0$, where $\varphi_{k}(\cdot; \mu)$ are the orthonormal polynomials with respect to the measure $\mu$. Let the positive measure $\nu$ on the unit circle be given by $d \nu(z) = |G_{2m}(z)|\, d \mu(z)$, where $G_{2m}$ is a conjugate reciprocal polynomial of exact degree $2m$. We establish a determinantal formula expressing $\{K_n(z,w;\nu)\}_{n \geq 0}$ directly in terms of $\{K_n(z,w;\mu)\}_{n \geq 0}$. Furthermore, we consider the special case of $w=1$; it is known that appropriately normalized polynomials $K_n(z,1;\mu) $ satisfy a recurrence relation whose coefficients are given in terms of two sets of real parameters $ \{c_n(\mu)\}_{n=1}^{\infty}$ and $ \{g_{n}(\mu)\}_{n=1}^{\infty}$, with $0<g_n<1 $ for $n\geq 1$. The double sequence $\{(c_n(\mu), g_{n}(\mu))\}_{n=1}^{\infty}$ characterizes the measure $\mu$. A natural question about the relation between the parameters $c_n(\mu)$, $g_n(\mu)$, associated with $\mu$, and the sequences $c_n(\nu)$, $g_n(\nu)$, corresponding to $\nu$, is also addressed. Finally, examples are considered, such as the Geronimus weight (a measure supported on an arc of the unit circle), a class of measures given by basic hypergeometric functions, and a class of measures with hypergeometric orthogonal polynomials.