Representation of conformal maps by rational functions

TL;DR

This paper introduces a fast and simple method for representing conformal maps using rational functions computed by the AAA algorithm, supported by theoretical convergence proofs and practical effectiveness for polygons and smooth domains.

Contribution

It proves root-exponential convergence of rational approximations near corners and develops a new algorithm for conformal maps, outperforming traditional contour integral methods.

Findings

Rational approximations converge root-exponentially near corners.

The new algorithm is 10-1000 times faster than traditional methods.

Effective for both polygonal and smooth domains.

Abstract

The traditional view in numerical conformal mapping is that once the boundary correspondence function has been found, the map and its inverse can be evaluated by contour integrals. We propose that it is much simpler, and 10-1000 times faster, to represent the maps by rational functions computed by the AAA algorithm. To justify this claim, first we prove a theorem establishing root-exponential convergence of rational approximations near corners in a conformal map, generalizing a result of D. J. Newman in 1964. This leads to the new algorithm for approximating conformal maps of polygons. Then we turn to smooth domains and prove a sequence of four theorems establishing that in any conformal map of the unit circle onto a region with a long and slender part, there must be a singularity or loss of univalence exponentially close to the boundary, and polynomial approximations cannot be accurate unless of exponentially high degree. This motivates the application of the new algorithm to smooth domains, where it is again found to be highly effective.