Bereanu, Cristian. “On a Multiplicity Result of J. R. Ward for Superlinear Planar Systems”. Topological Methods in Nonlinear Analysis 27, no. 2 (June 1, 2006): 289–298. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2006.012.