HANSEN, Frank; OLESEN, Dorte. Perturbations of centre-fixing dynamical systems. MATHEMATICA SCANDINAVICA, [S. l.], v. 41, p. 295–307, 1977. DOI: 10.7146/math.scand.a-11722. Disponível em: https://www.mscand.dk/article/view/11722. Acesso em: 5 dec. 2025.