Bodraya, V. I., O. A. Novikov, and V. I. Rukasov. 2005. “Approximation of Classes of ψ-Integrals of Periodic Functions of Many Variables by Rectangular Linear Means of Their Fourier Series”. Ukrains’kyi Matematychnyi Zhurnal 57 (4): 564–570. https://umj.imath.kiev.ua/index.php/umj/article/view/3622.