Shivaji, R., and B. Son. “Bifurcation and Multiplicity Results for Classes of $p,q$-Laplacian Systems”. Topological Methods in Nonlinear Analysis, vol. 48, no. 1, Apr. 2016, pp. 103-14, https://apcz.umk.pl/TMNA/article/view/TMNA.2016.036.