Agarwal, R. P., Filippakis, M. E., O‘Regan, D., & Papageorgiou, N. S. (2008). Multiple Solutions for Doubly Resonant Elliptic Problems Using Critical Groups. CUBO, A Mathematical Journal, 10(3), 21–41. Retrieved from https://cubo.ufro.cl/ojs/index.php/cubo/article/view/1497