SerdyukA. S. “On the Best Approximation of Classes of Convolutions of Periodic Functions by Trigonometric Polynomials”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 47, no. 9, Sept. 1995, pp. 1261–1265, https://umj.imath.kiev.ua/index.php/umj/article/view/5526.