Dimension distortion by Sobolev mappings on the Heisenberg group

The Heisenberg group is equipped with a foliation by horizontal lines that differs substantially from the standard foliation of Euclidean space by lines. We investigate the behavior of Sobolev mappings on this foliation. It is a fundamental property of Sobolev mappings that they are absolutely continuous on almost every line; we estimate the quantity of lines whose image under a Sobolev mapping has dimension at least a fixed number d>1.