@Article{ albamb96, author = { Alberti, Giovanni and Ambrosio, Luigi }, title = { A Geometrical Approach to Monotone Functions in ${\mathbb R}^n$ }, journal = { Math. Zeit. }, year = { 1996 }, volume = { 230 (1999) }, pages = { 259-316 }, URL = { http://cvgmt.sns.it/paper/236/ }, note = { cvgmt preprint} }