Clapp, Mónica, and Gustavo Izquierdo. “Multiple Positive Symmetric Solutions of a Singularly Perturbed Elliptic Equation”. Topological Methods in Nonlinear Analysis 18, no. 1 (September 1, 2001): 17–39. Accessed April 19, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2001.022.