BEREANU, C. On a multiplicity result of J. R. Ward for superlinear planar systems. Topological Methods in Nonlinear Analysis, [S. l.], v. 27, n. 2, p. 289–298, 2006. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2006.012. Acesso em: 27 apr. 2024.