CHE, Guofeng and WU, Tsung-fang. Three positive solutions for the indefinite fractional Schrödinger-Poisson systems. Topological Methods in Nonlinear Analysis. Online. 23 September 2023. Vol. 62, no. 1, pp. 53 - 81. [Accessed 22 July 2024]. DOI 10.12775/TMNA.2022.046.