@article{AFM_2008_33_1_a1,
     author = {Nicola Arcozzi and Fausto Ferrari},
     title = {The {Hessian} of the distance from a surface in the {Heisenberg} group},
     journal = {Annales Fennici Mathematici},
     pages = {35--63},
     publisher = {mathdoc},
     volume = {33},
     number = {1},
     year = {2008},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/AFM_2008_33_1_a1/}
}