Choudhury, B. S., and A. Kundu. “FIXED POINT THEOREMS FOR NONLINEAR CONTRACTIONS IN ORDERED PARTIAL METRIC SPACES”. Journal of Nonlinear Analysis and Optimization: Theory & Applications, vol. 4, no. 1, Oct. 2012, pp. 147-58, http://www.math.sci.nu.ac.th/ojs302/index.php/jnao/article/view/187.