@Misc{ GerMonMor18, author = { Gerosa, Daniele and Monti, Roberto and Morbidelli, Daniele }, title = { A trace theorem for Martinet-type vector fields }, journal = { Cont. Math. }, year = { 2018 }, pages = { }, URL = { http://cvgmt.sns.it/paper/4095/ }, note = { cvgmt preprint} }