Iqbal, Iram, and Nawab Hussain. “Ekeland-Type Variational Principle With Applications to Nonconvex Minimization and Equilibrium Problems”. Nonlinear Analysis: Modelling and Control, vol. 24, no. 3, Apr. 2019, pp. 407-32, https://doi.org/10.15388/NA.2019.3.6.