WAHDE, G. A Closure Problem for Spaces of Infinitely Differentiable Functions. MATHEMATICA SCANDINAVICA, [S. l.], v. 20, p. 209–224, 1967. DOI: 10.7146/math.scand.a-10831. Disponível em: https://www.mscand.dk/article/view/10831. Acesso em: 3 jul. 2024.