1. J. Heinonen, Lectures on analysis on metric spaces, Universitext, Springer-Verlag, New York, 2001.
2. A. Bonfiglioli, E. Lanconelli and F. Uguzzoni, Stratified Lie groups and potential theory for their sub-Laplacians, Springer Monographs in Mathematics, vol. 26, Springer-Verlag, New York, 2007.
3. Z. Buczolich, Solution of the gradient problem of C.E. Weil, Rev. Math. Ibero. 21 (2005), no. 3, 889–910.
4. L. Capogna, D. Danielli, S. Pauls and J. Tyson, An introduction to the Heisenberg group and the sub-Riemannian isoperimetric problem, Progress in Mathematics, vol. 259, Birkhauser, Basel, 2007.
5. G. Citti, M. Manfredini, A. Pinamonti and F. Serra Cassano, Smooth approximation for intrinsic Lipschitz functions in the Heisenberg group, Calc. Var. Partial Differential Equations 49 (2014), no. 3–4, 1279–1308.