STETKAER, Henrik. Invariant Pseudo-Differential Operators. Mathematica Scandinavica, Berkeley, CA, United States, v. 28, p. 105–123, 1971. DOI: 10.7146/math.scand.a-11009. Disponível em: https://journals.msp.org/mscand/article/view/2287. Acesso em: 25 jul. 2026.