# Vertical versus horizontal Sobolev spaces

## TL;DR

This paper demonstrates that horizontal Sobolev spaces on the Heisenberg group are contained in vertical Sobolev spaces without localization, leading to new insights into vertical derivatives of Lipschitz functions and Poincaré inequalities.

## Contribution

It establishes a sharper inclusion of horizontal into vertical Sobolev spaces on the Heisenberg group, removing localization requirements and enabling new applications.

## Key findings

- Bounded Lipschitz functions have a 1/2-order vertical derivative in BMO.
- Provides a fractional order generalization of vertical vs. horizontal Poincaré inequalities.
- Shows continuous embedding of horizontal Sobolev spaces into vertical Sobolev spaces without localization.

## Abstract

Let $\alpha \geq 0$, $1 < p < \infty$, and let $\mathbb{H}^{n}$ be the Heisenberg group. Folland in 1975 showed that if $f \colon \mathbb{H}^{n} \to \mathbb{R}$ is a function in the horizontal Sobolev space $S^{p}_{2\alpha}(\mathbb{H}^{n})$, then $\varphi f$ belongs to the Euclidean Sobolev space $S^{p}_{\alpha}(\mathbb{R}^{2n + 1})$ for any test function $\varphi$. In short, $S^{p}_{2\alpha}(\mathbb{H}^{n}) \subset S^{p}_{\alpha,\mathrm{loc}}(\mathbb{R}^{2n + 1})$. We show that the localisation can be omitted if one only cares for Sobolev regularity in the vertical direction: the horizontal Sobolev space $S_{2\alpha}^{p}(\mathbb{H}^{n})$ is continuously contained in the vertical Sobolev space $V^{p}_{\alpha}(\mathbb{H}^{n})$. Our search for the sharper result was motivated by the following two applications. First, combined with a short additional argument, it implies that bounded Lipschitz functions on $\mathbb{H}^{n}$ have a $\tfrac{1}{2}$-order vertical derivative in $\mathrm{BMO}(\mathbb{H}^{n})$. Second, it yields a fractional order generalisation of the (non-endpoint) vertical versus horizontal Poincar\'e inequalities of V. Lafforgue and A. Naor.