BoichukО. A., ZhuravlevV. F., and PokutnyiО. О. 2013. “Normally Solvable Operator Equations in a Banach Space”. Ukrains’kyi Matematychnyi Zhurnal 65 (2), 163-74. https://umj.imath.kiev.ua/index.php/umj/article/view/2411.