@Article{ dacmarpao09, author = { Dacorogna, Bernard and Marcellini, Paolo and Paolini, Emanuele }, title = { Functions with orthogonal Hessian }, journal = { Differential and Integral Equations }, year = { 2010 }, volume = { 23 }, number = { 1--2 }, pages = { 51-60 }, URL = { http://cvgmt.sns.it/paper/1043/ }, note = { cvgmt preprint} }