Sastry, K. P. R. ., S. V. R. Naidu, G. V. R. Babu, and G. A. . Naidu. “Generalization of Common Fixed Point Theorems for Weakly Commuting Maps by Altering Distances”. Tamkang Journal of Mathematics, vol. 31, no. 3, Sept. 2000, pp. 243-50, doi:10.5556/j.tkjm.31.2000.399.