BOMAN, Jan. The sum of two plane convex $C^\infty$ sets is not always $C^5$. Mathematica Scandinavica, Berkeley, CA, United States, v. 66, p. 216–224, 1990. DOI: 10.7146/math.scand.a-12305. Disponível em: https://journals.msp.org/mscand/article/view/1315. Acesso em: 25 jul. 2026.