Hirata, J., N. Ikoma, and K. Tanaka. “Nonlinear Scalar Field Equations in $\mathbb{R}^{N}$: Mountain Pass and Symmetric Mountain Pass Approaches”. Topological Methods in Nonlinear Analysis, vol. 35, no. 2, Apr. 2010, pp. 253-76, https://apcz.umk.pl/TMNA/article/view/TMNA.2010.015.