Duke Mathematical Journal
We show that if f is a 1-quasiconformal map defined on an open subset of a Carnot group G, then composition with f preserves Q-harmonic functions. We combine this with a regularity theorem for Q-harmonic functions and an algebraic regularity theorem for maps between Carnot groups to show that f is smooth. We give some applications to the study of rigidity.
Capogna, Luca and Cowling, Michael, "Conformality and Q-harmonicity in Carnot Groups" (2006). Mathematics and Statistics: Faculty Publications, Smith College, Northampton, MA.