Voloshyn, H.A., and V.K. Maslyuchenko. “On Approximation of the Separately Continuous Functions $2\pi$-Periodical in Relation to the Second Variable”. Carpathian Mathematical Publications, vol. 2, no. 1, June 2010, pp. 4-14, https://journals.pnu.edu.ua/index.php/cmp/article/view/3975.