Lin, Huei-li, Hwai-chiuan Wag, and Tsung-fang Wu. 2002. “A Palais-Smale Approach to Sobolev Subcritical Operators”. Topological Methods in Nonlinear Analysis 20 (2):393-407. https://apcz.umk.pl/TMNA/article/view/TMNA.2002.042.