Berkovits, J., H. Leinfelder, and V. Mustonen. “Existence and Multiplicity Results for Wave Equations With Time-Independent Nonlinearity”. Topological Methods in Nonlinear Analysis, vol. 22, no. 2, Dec. 2003, pp. 273-95, https://apcz.umk.pl/TMNA/article/view/TMNA.2003.040.