BOMAN, J. The sum of two plane convex $C^\infty$ sets is not always $C^5$. MATHEMATICA SCANDINAVICA, [S. l.], v. 66, p. 216–224, 1990. DOI: 10.7146/math.scand.a-12305. Disponível em: https://www.mscand.dk/article/view/12305. Acesso em: 14 apr. 2025.