CLAESSON, Tomas. Partial Hypoellipticity and Gevrey Classes. Mathematica Scandinavica, Berkeley, CA, United States, v. 26, p. 119–131, 1970. DOI: 10.7146/math.scand.a-10971. Disponível em: https://journals.msp.org/mscand/article/view/2355. Acesso em: 25 jul. 2026.