Alves, C. O. “Multiplicity of Multi-Bump Type Nodal Solutions for a Class of Elliptic Problems in ${\Bbb R}^N$”. Topological Methods in Nonlinear Analysis, vol. 34, no. 2, Dec. 2009, pp. 231-50, https://apcz.umk.pl/TMNA/article/view/TMNA.2009.040.