WAHDE, Gösta. A Closure Problem for Spaces of Infinitely Differentiable Functions. Mathematica Scandinavica, Berkeley, CA, United States, v. 20, p. 209–224, 1967. DOI: 10.7146/math.scand.a-10831. Disponível em: https://journals.msp.org/mscand/article/view/2495. Acesso em: 25 jul. 2026.