Rao, K., G. Kishore, and N. Van Luong. “A Unique Common Coupled Fixed Point Theorem for Four Maps under ψ - φ Contractive Condition in Partial Metric Spaces”. CUBO, A Mathematical Journal, vol. 14, no. 3, Oct. 2012, pp. 115–127, doi:10.4067/S0719-06462012000300008.