A Complete Characterization of Pythagorean Hodograph Preserving Mappings

Abstract

We fully characterize the mappings $\Phi$ that send every Pythagorean-hodograph (PH) curve to a PH curve. We prove that in any dimension, such mappings are precisely the conformal functions whose dilation is the square of a real rational function. In the planar case, this implies (up to conjugation) that $\partial \Phi /\partial z={\Psi }^2$, where $\Psi$ is meromorphic and satisfies $\mathrm{Res}({\Psi }^2)=0$ at every pole. In higher dimensions, PH preservation forces $\Phi$ to be a conformal map; for $n\ge 3$, Liouville's theorem then implies that any local diffeomorphism with this property is (anti-)Möbius. These results subsume the previously known ``(scaled) PH-preserving'' constructions of mappings ${\mathbb{R}}^2\to {\mathbb{R}}^3$ and align with Ueda's conformal viewpoint on isothermal and spherical geometries. At the level of examples, we demonstrate how PH-preserving mappings relate to the construction of rational PH curves and minimal surfaces.

arXiv commentAccepted in Proceedings of the Royal Society A