@Misc{ CiMaPiSe12, author = { Citti, Giovanna and Manfredini, Maria and Pinamonti, Andrea and Serra Cassano, Francesco }, title = { Smooth approximation for intrinsic Lipschitz functions in the Heisenberg group }, journal = { Calc. Var. Partial Differential Equations }, year = { 2013 }, pages = { }, URL = { http://cvgmt.sns.it/paper/1760/ }, note = { cvgmt preprint} }