A Funk perspective on billiards, projective geometry and Mahler volume

TL;DR

This paper investigates the properties of the Funk metric and its invariants, explores Funk billiards, and proposes affine inequalities related to convex geometry, extending classical results and introducing new invariants and conjectures.

Contribution

It introduces new invariants and conjectures in Funk geometry, extends duality results for Funk billiards, and provides a new proof of the volume entropy conjecture for unconditional bodies.

Findings

Many Funk metric invariants are projectively invariant.

Funk billiards generalize hyperbolic billiards and relate to Minkowski billiards.

A new affine inequality conjecture encompasses classical inequalities, proven for unconditional bodies.

Abstract

We explore connections furnished by the Funk metric, a relative of the Hilbert metric, between projective geometry, billiards, convex geometry and affine inequalities. We first show that many metric invariants of the Funk metric are invariant under projective transformations as well as projective duality. These include the Holmes-Thompson volume and surface area of convex subsets, and the length spectrum of their boundary, extending results of Holmes-Thompson and \'Alvarez Paiva on Sch\"affer's dual girth conjecture. We explore in particular Funk billiards, which generalize hyperbolic billiards in the same way that Minkowski billiards generalize Euclidean ones, and extend a result of Gutkin-Tabachnikov on the duality of Minkowski billiards. We next consider the volume of outward balls in Funk geometry. We conjecture a general affine inequality corresponding to the volume maximizers, which includes the Blaschke-Santal\'o and centro-affine isoperimetric inequalities as limit cases, and prove it for unconditional bodies, yielding a new proof of the volume entropy conjecture for the Hilbert metric for unconditional bodies. As a by-product, we obtain generalizations to higher moments of inequalities of Ball and Huang-Li, which in turn strengthen the Blaschke-Santal\'o inequality for unconditional bodies. Lastly, we introduce a regularization of the total volume of a smooth strictly convex 2-dimensional set equipped with the Funk metric, resembling the O'Hara M\"obius energy of a knot, and show that it is a projective invariant of the convex body.