Afrouzi, G., Q. Zhang, and Z. Naghizadeh. “BANACH FIXED POINT THEOREM IN APPLICATION TO NONLINEAR ELLIPTIC SYSTEMS”. Journal of Nonlinear Analysis and Optimization: Theory & Applications, vol. 4, no. 2, June 2013, pp. 149-55, http://www.math.sci.nu.ac.th/ojs302/index.php/jnao/article/view/172.