ARCOZZI, N.; FERRARI, F. The Hessian of the distance from a surface in the Heisenberg group. Annales Fennici Mathematici, [S. l.], v. 33, n. 1, p. 35–63, 2008. Disponível em: https://afm.journal.fi/article/view/135215. Acesso em: 11 apr. 2025.