BoichukО. A., V. F. Zhuravlev, and PokutnyiО. О. “Normally Solvable Operator Equations in a Banach Space”. Ukrains’kyi Matematychnyi Zhurnal 65, no. 2 (February 25, 2013): 163-174. Accessed January 28, 2023. https://umj.imath.kiev.ua/index.php/umj/article/view/2411.