AndroshchukA. A. “On Transformation Operators for a Second Order Differential Equation With Operator Coefficients ”. Ukrains’kyi Matematychnyi Zhurnal 23, no. 1 (December 25, 1970): 52-55. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/8479.