Change of Basis from Bernstein to Zernike

TL;DR

This paper develops a comprehensive framework for changing bases between polynomial families like Bernstein and Zernike, introducing new techniques, proving properties, and solving open problems in the process.

Contribution

It defines ascending and descending bases, introduces techniques for basis transformation, and provides new formulas and solutions for change of basis problems involving Bernstein and Zernike polynomials.

Findings

Established coefficient functions for basis mappings

Proved the non-existence of a closed-form solution for a specific basis change

Connected change of basis matrices to category theory concepts

Abstract

We increase the scope of previous work on change of basis between finite bases of polynomials by defining ascending and descending bases and introducing three techniques for defining them from known ones. The minimum degrees of polynomials in an ascending basis can increase such as with bases of Bernstein and Zernike Radial polynomials. They have applications in computer-aided design and optics. We give coefficient functions for mappings from the monomials to descending bases of Bernstein polynomials, and ascending ones of Zernike Radial polynomials and prove their correctness. Allowing for parity, we define eight general change of basis matrices and the related equations for composing their coefficient functions. A main example is the change of basis from shifted Legendre polynomials to Bernstein polynomials considered by R Farouki [7]. The analysis enables us to find a more general hypergeometric function for the coefficient function, and recurrences for finding coefficient functions by matrix row and column. We show the column coefficient functions are equivalent to the Lagrange interpolation polynomials of their elements. We also provide a solution to an open problem [7] by showing there is no general closed form expression for the coefficient function from Gosper's Algorithm. Truncation, alternation and superposition increase the scope further. Alternation enables, for example, a change of basis between Bernstein and Zernike Radial polynomials. A summary shows that the groupoid of change of basis matrices is a small category, and triangular and alternating change of basis matrices are morphisms in full subcategories of it. Truncation is a covariant functor.