Chen, Chao-Ping, and Feng Qi. “A Double Inequality for Remainder of Power Series of Tangent Function”. Tamkang Journal of Mathematics 34, no. 4 (December 31, 2003): 351–356. Accessed July 3, 2024. https://journals.math.tku.edu.tw/index.php/TKJM/article/view/236.