BoichukО. A., and PokutnyiО. О. “Perturbation Theory of Operator Equations in the FréChet and Hilbert Spaces”. Ukrains’kyi Matematychnyi Zhurnal 67, no. 9 (September 25, 2015): 1181-1188. Accessed April 20, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2057.