Aswhad, A. A. (2017). On the Riesz Means of Expansion by Riesz Bases Formed by Eigen Functions for the Ordinary Differential Operator of 2mth Order. Ibn AL-Haitham Journal For Pure and Applied Sciences, 23(2), 159-167. https://jih.uobaghdad.edu.iq/index.php/j/article/view/1017