Du, Yihong, and Yuxia Guo. “Mountain Pass Solutions and an Indefinite Superlinear Elliptic Problem on $\mathbb R^{\mathbb N}$”. Topological Methods in Nonlinear Analysis 22, no. 1 (September 1, 2003): 69–92. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2003.028.