Hasan, Shatha, and Mona Sakkijha. “Toward Strictly Singular Fractional Operator Restricted by Fredholm-Volterra in Sobolev Space”. Italian Journal of Pure and Applied Mathematics, vol. 43, Feb. 2020, pp. 416-27, https://journals.uniurb.it/index.php/ijpam/article/view/7330.