Insight in the Rumin Cohomology and Orientability Properties of the Heisenberg Group

TL;DR

This paper explores the Rumin cohomology and $ ext{H}$-orientability in the Heisenberg group, establishing new links between differential forms, orientability, and the existence of non-orientable surfaces within this geometric setting.

Contribution

It introduces the concept of $ ext{H}$-orientability for $ ext{H}$-regular surfaces and demonstrates the existence of non-$ ext{H}$-orientable surfaces in the Heisenberg group, expanding understanding of geometric structures.

Findings

Rumin cohomology detailed with examples for n=1,2

H-orientability implies standard orientability, but not vice versa

Existence of non-H-orientable H-regular surfaces in H^1

Abstract

The purpose of this study is to analyse two related topics: the Rumin cohomology and the $\mathbb{H}$-orientability in the Heisenberg group $\mathbb{H}^n$. In the first three chapters we carefully describe the Rumin cohomology with particular emphasis at the second order differential operator $D$, giving examples in the cases $n=1$ and $n=2$. We also show the commutation between all Rumin differential operators and the pullback by a contact map and, more generally, describe pushforward and pullback explicitly in different situations. Differential forms can be used to define the notion of orientability; indeed in the fourth chapter we define the $\mathbb{H}$-orientability for $\mathbb{H}$-regular surfaces and we prove that $\mathbb{H}$-orientability implies standard orientability, while the opposite is not always true. Finally we show that, up to one point, a M\"obius strip in $\mathbb{H}^1$ is a $\mathbb{H}$-regular surface and we use this fact to prove that there exist $\mathbb{H}$-regular non-$\mathbb{H}$-orientable surfaces, at least in the case $n=1$. This opens the possibility for an analysis of Heisenberg currents mod $2$.