CLAESSON, T. Partial Hypoellipticity and Gevrey Classes. MATHEMATICA SCANDINAVICA, [S. l.], v. 26, p. 119–131, 1970. DOI: 10.7146/math.scand.a-10971. Disponível em: https://www.mscand.dk/article/view/10971. Acesso em: 5 may. 2024.