M. G. Volkova and G. M. Gubreev, “One remark about the unconditional exponential bases and cosine bases, connected with them”, MFAT, vol. 14, no. 4, pp. 330–333, Dec. 2008, Accessed: Feb. 04, 2026. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/396