Hölder maps under Pfaffian constraints

Abstract

Given a one form $λ$ in $\mathbb{R}^N$ and $f: \mathbb{S}^{n} \to \mathbb{R}^N$ with $f^\ast λ= 0$ we discuss the maximal Hölder regularity of extensions $F: \mathbb{B}^{n+1} \to \mathbb{R}^N$ such that $F^\astλ= 0$ in distributional sense. Our analysis applies to the Heisenberg groups $\mathbb{H}_n$. It implies in particular that for all $n \geq 1$ any smooth horizontal map $f: \mathbb{S}^{n} \to \mathbb{H}_n$ can be extended to a $C^α$-map $F: \mathbb{B}^{n+1} \to \mathbb{H}_n$ for some $α> 1/2$. Moreover, if $n \geq 3$ we find $C^α$-embeddings from $\mathbb{B}^{n+1}$ into $\mathbb{H}_n$ for some $α> \frac{1}{2}$. In the appendix we discuss an (as of now unverified) approach to extend these arguments to find $C^α$-embeddings from $\mathbb{B}^{2}$ into $\mathbb{H}_1$ for some $α> \frac{1}{2}$, assisted by GPT-6 Astra.