WÓJCIK, Paweł. $k$-smoothness: an answer to an open problem. Mathematica Scandinavica, Berkeley, CA, United States, v. 123, n. 1, p. 85–90, 2018. DOI: 10.7146/math.scand.a-102834. Disponível em: https://journals.msp.org/mscand/article/view/273. Acesso em: 24 jul. 2026.