Ponomarenko, Yu. A., and Tіman M. F. “On Absolute Summability of Multiple Fourier Series ”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 23, no. 3, Apr. 1971, pp. 346-61,
https://umj.imath.kiev.ua/index.php/umj/article/view/8561
.