RomanyukA. S., SerdyukA. S., and StepanetsO. I. “The Second School ‘Fourier Series. Theory and Applications’”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 49, no. 11, Nov. 1997, p. 1584, https://umj.imath.kiev.ua/index.php/umj/article/view/5161.