Moghaddam, M., and T. Sistani. “Best Approximation for Convex Subsets of 2-Inner Product Spaces”. Journal of Nonlinear Analysis and Optimization: Theory & Applications 3, no. 2 (September 30, 2012): 269-278. Accessed December 9, 2021. http://www.math.sci.nu.ac.th/ojs302/index.php/jnao/article/view/166.