Constructing H\"older maps to Carnot groups

Abstract

In this paper, we construct H\"older maps to Carnot groups equipped with a Carnot metric, especially the first Heisenberg group $\mathbb{H}$. Pansu and Gromov observed that any surface embedded in $\mathbb{H}$ has Hausdorff dimension at least 3, so there is no $\alpha$-H\"older embedding of a surface into $\mathbb{H}$ when $\alpha>\frac{2}{3}$. Z\"ust improved this result to show that when $\alpha>\frac{2}{3}$, any $\alpha$-H\"older map from a simply-connected Riemannian manifold to $\mathbb{H}$ factors through a metric tree. In the present paper, we show that Z\"ust's result is sharp by constructing $(\frac{2}{3}-\epsilon)$-H\"older maps from $D^2$ and $D^3$ to $\mathbb{H}$ that do not factor through a tree. We use these to show that if $0<\alpha < \frac{2}{3}$, then the set of $\alpha$-H\"older maps from a compact metric space to $\mathbb{H}$ is dense in the set of continuous maps and to construct proper degree-1 maps from $\mathbb{R}^3$ to $\mathbb{H}$ with H\"older exponents arbitrarily close to $\frac{2}{3}$.