Benci, Vieri, et al. “Existence and Non Existence of the Ground State Solution for the Nonlinear Schroedinger Equations With $V(\infty)=0$”. Topological Methods in Nonlinear Analysis, vol. 26, no. 2, Dec. 2005, pp. 203-19, https://apcz.umk.pl/TMNA/article/view/TMNA.2005.031.