HANSEN, Frank; OLESEN, Dorte. Perturbations of centre-fixing dynamical systems. Mathematica Scandinavica, Berkeley, CA, United States, v. 41, p. 295–307, 1977. DOI: 10.7146/math.scand.a-11722. Disponível em: https://journals.msp.org/mscand/article/view/1925. Acesso em: 25 jul. 2026.