BROOKS, R. M. Boundaries for Natural Systems II. Mathematica Scandinavica, Berkeley, CA, United States, v. 30, p. 281–289, 1972. DOI: 10.7146/math.scand.a-11084. Disponível em: https://journals.msp.org/mscand/article/view/2243. Acesso em: 25 jul. 2026.