“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, vol. 22, no. 1, Aug. 2017, https://jih.uobaghdad.edu.iq/index.php/j/article/view/1277.