Hwang, Dah-Yan, and Gou-Sheng Yang. “Pre-Gr"uss Type Inequalities in $ 2 $-Inner Product Spaces”. Tamkang Journal of Mathematics 38, no. 3 (September 30, 2007): 191–204. Accessed January 31, 2025. https://journals.math.tku.edu.tw/index.php/TKJM/article/view/71.