An explicit exotic representation of a rank-one simple Lie group via convex bodies

TL;DR

This paper constructs a new irreducible representation of PSL(2,R) acting on an infinite-dimensional hyperbolic space using convex bodies, revealing novel geometric and topological properties of the action.

Contribution

It introduces an explicit convex-body-based representation of PSL(2,R) on infinite-dimensional hyperbolic space, expanding understanding of exotic group actions.

Findings

Existence of a continuous irreducible representation of PSL(2,R) on H^∞

The quotient space is homeomorphic to the Banach–Mazur compactum

Computed the Hausdorff dimension of the limit set

Abstract

In [DP12], Delzant and Py showed that there exist continuous irreducible isometric actions of $\mathrm{PSL}_2(\mathbb{R})$ on the infinite-dimensional hyperbolic space $\mathbb{H}^\infty$. Such continuous irreducible actions do not exist on the hyperbolic spaces $\mathbb{H}^n$ when $n>2$ and their associated embeddings $\mathbb{H}^2 \to \mathbb{H}^\infty$ given by the orbit maps were later called exotic by Monod and Py in [MP14]. In this article, we produce a continuous and irreducible representation of $\mathrm{PSL}_2(\mathbb{R})\to \mathrm{Isom}(\mathbb{H}^\infty)$ using the hyperbolic model for convex bodies introduced in [DF22]. This yields a convex cocompact $\mathrm{PSL}_2(\mathbb{R})$-action on the infinite-dimensional hyperbolic space $\mathbb{H}^\infty$, of which the compact quotient over the minimal $\mathrm{PSL}_2(\mathbb{R})$-invariant convex set is homeomorphic to the 2-dimensional oriented Banach--Mazur compactum. Moreover, we study the geometry of one of its orbit maps and compute the Hausdorff dimension of the limit set of this representation.