Babenko, V. V., V. F. Babenko, and M. V. Polishchuk. 2016. “Approximation of Some Classes of Set-Valued Periodic Functions by Generalized Trigonometric Polynomials”.
Ukrains’kyi Matematychnyi Zhurnal 68 (4): 449-59.
https://umj.imath.kiev.ua/index.php/umj/article/view/1851.