KochmanskiS. “On the Evolution Operators for Some Equations of Mathematical Physics With Variable Coefficients”. Ukrains’kyi Matematychnyi Zhurnal 46, no. 7 (July 25, 1994): 857–869. Accessed October 6, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5677.