BoichukО. A., & PokutnyiО. О. (2015). Perturbation Theory of Operator Equations in the FréChet and Hilbert Spaces. Ukrains’kyi Matematychnyi Zhurnal, 67(9), 1181-1188. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/2057