Voulov, H. D., M. J. Bohner, and M. B. Bekker. “A Q-Difference Operator With Discrete and Simple Spectrum”. Methods of Functional Analysis and Topology 17, no. 4 (December 25, 2011): 281–294. Accessed February 4, 2026. https://zen.imath.kiev.ua/index.php/mfat/article/view/493.