Ambrosio, Vincenzo. 2016. “Periodic Solutions for the Non-Local Operator $(-\Delta+m^{2})^{s}-m^{2s}$ With $m\geq 0$”. Topological Methods in Nonlinear Analysis 49 (1):75-103. https://apcz.umk.pl/TMNA/article/view/TMNA.2016.063.