@Article{ RizSer16, author = { Rizzi, Luca and Serres, Ulysse }, title = { On the cut locus of free, step two Carnot groups }, journal = { Proceedings of the AMS }, year = { 2017 }, doi = { 10.1090/proc/13658 }, pages = { }, URL = { http://cvgmt.sns.it/paper/3191/ }, note = { cvgmt preprint} }