Smoothness of Lipschitz minimal intrinsic graphs in Heisenberg groups   $H^n, n>1$
  We prove that Lipschitz intrinsic graphs in the Heisenberg groups $H^n$, with $n>1$, which are vanishing viscosity solutions of the minimal surface equation are smooth. 
Luca Capogna, Giovanna Citti, Maria Manfredini