1. J. G. Azorero and I. Peral Alonso, ‘‘Hardy inequalities and some critical elliptic and parabolic problems,’’ J. Differ. Equat. 144, 441–476 (1998).
2. G. A. Afrouzi, A. Hadjian, and Sh. Heidarkhani, ‘‘Steklov problems involving the $$p(x)$$-Laplacian,’’ Electron. J. Differ. Equat., No. 134, 1–11 (2014).
3. L. Barbu and G. Moroşanu, ‘‘Eigenvalues of the negative $$(p,q)$$-Laplacian under a Steklov like boundary condition,’’ Complex Var. Ellipt. Equat. 64, 685–700 (2019).
4. G. Bonanno, ‘‘A critical point theorem via the Ekeland variational principle,’’ Nonlin. Anal. 75, 2929–3007 (2012).
5. G. Bonanno, ‘‘Relation between the mountain pass theorem and local minima,’’ Adv. Nonlin. Anal. 1, 205–220 (2012).