S. A. Pichugov, “Approximation of measurable periodic functions in measure by step functions”, Ukr. Mat. Zhurn., vol. 48, no. 5, pp. 711–715, May 1996, Accessed: Apr. 09, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5305