A Funk perspective on billiards, projective geometry and Mahler volume

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 maps, as well as under 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 Álvarez Paiva on Schäffer’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–Santaló 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–Santaló 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 Möbius energy of a knot, and show that it is a projective invariant of the convex body.