Kuznetsovci, O. I. “Strong Summability of Multiple Fourier Series and Sidon-Type Inequalities”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 12, Dec. 1998, pp. 1630–1635, https://umj.imath.kiev.ua/index.php/umj/article/view/4782.