A. S. Serdyuk, O. I. Stepanets, and A. L. Shydlich, “On some new criteria for infinite differentiability of periodic functions”, Ukr. Mat. Zhurn., vol. 59, no. 10, pp. 1399–1409, Oct. 2007, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3398