On the Characterization of p-Harmonic Functions on the Heisenberg Group   by Mean Value Properties
  We characterize $p-$harmonic functions in the Heisenberg group in terms of an asymptotic mean value property, where $1<p<\infty$, following the scheme described in Manfredi et al. (2009) for the Euclidean case. The new tool that allows us to consider the subelliptic case is a geometric lemma, Lemma 3.2 below, that relates the directions of the points of maxima and minima of a function on a small subelliptic ball with the unit horizontal gradient of that function. 
Fausto Ferrari, Qing Liu, Juan J. Manfredi
math.AP