@article{article08ba15e9,
title   = {Global analytic regularity for sums of squares of vector fields},
author = {Cordaro, Paulo Domingos and Himonas, Alexandros Alexandrou},
year = {1998},
doi = {10.1090/S0002-9947-98-01987-4},
journal   = {Transactions of the American Mathematical Society}
}