Boichuk О.A. and Pokutnyi О.О. (2015) “Perturbation Theory of Operator Equations in the FréChet and Hilbert Spaces”, Ukrains’kyi Matematychnyi Zhurnal, 67(9), pp. 1181–1188. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/2057 (Accessed: 17 February 2026).