Babenko, V. V., V. F. Babenko, and M. V. Polishchuk. “Approximation of Some Classes of Set-valued periodic Functions by Generalized Trigonometric Polynomials”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 68, no. 4, Apr. 2016, pp. 449-5, https://umj.imath.kiev.ua/index.php/umj/article/view/1851.