Volkova, M. G., and G. M. Gubreev. “One Remark about the Unconditional Exponential Bases and Cosine Bases, Connected With Them”. Methods of Functional Analysis and Topology 14, no. 4 (December 25, 2008): 330–333. Accessed December 16, 2025. https://zen.imath.kiev.ua/index.php/mfat/article/view/396.