BROOKS, R. M. Boundaries for Natural Systems II. MATHEMATICA SCANDINAVICA, [S. l.], v. 30, p. 281–289, 1972. DOI: 10.7146/math.scand.a-11084. Disponível em: https://www.mscand.dk/article/view/11084. Acesso em: 3 may. 2024.