Serdyuk, A. S., O. I. Stepanets, and A. L. Shydlich. 2007. “On Some New Criteria for Infinite Differentiability of Periodic Functions”. Ukrains’kyi Matematychnyi Zhurnal 59 (10), 1399–1409. https://umj.imath.kiev.ua/index.php/umj/article/view/3398.