Tahir, M. B., and A. A. Aswhad. “On the Riesz Means of Expansions by Riesz Bases Formed by Eigenfunctions for the Ordinary Differential Operator of 4-Th Order”. Ibn AL-Haitham Journal For Pure and Applied Sciences 22, no. 1 (August 16, 2017). Accessed March 29, 2025. https://jih.uobaghdad.edu.iq/index.php/j/article/view/1277.