MaksimenkoS. I. “Period Functions for $\mathcal{C}^0$- and $\mathcal{C}^1$-Flows”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 62, no. 7, July 2010, pp. 954–967, https://umj.imath.kiev.ua/index.php/umj/article/view/2928.