N. A. Dehghan and H. Mazaheri, “New results in $G$-best approximation in $G$-metric spaces”, Ukr. Mat. Zhurn., vol. 62, no. 4, pp. 567–571, Apr. 2010, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/2888