Dimension distortion by right coset projections in the Heisenberg group

Abstract

We study the family of vertical projections whose fibers are right cosets of horizontal planes in the Heisenberg group, ${\mathbb{H}}^n$. We prove lower bounds for Hausdorff dimension distortion of sets under these mappings with respect to the natural quotient metric, which we show behaves like the Euclidean metric in this context. Our bounds are sharp in a large part of the dimension range, and we give conjectural sharp lower bounds for the remaining range. Our approach also lets us improve the known almost sure lower bound for the standard family of vertical projections in ${\mathbb{H}}^n$ for $n\ge 2$.