Yan, Shusen. “On the Number of Interior Multipeak Solutions for Singularly Perturbed Neumann Problems”. Topological Methods in Nonlinear Analysis 12, no. 1 (September 1, 1998): 61–78. Accessed March 29, 2023. https://apcz.umk.pl/TMNA/article/view/TMNA.1998.028.