EKELAND, I. Some lemmas about dynamical systems. Mathematica Scandinavica, Berkeley, CA, United States, v. 52, p. 262–268, 1983. DOI: 10.7146/math.scand.a-12005. Disponível em: https://journals.msp.org/mscand/article/view/1617. Acesso em: 25 jul. 2026.