M. L. Gorbachuk, “On the spectral functions of differential equations of the second order with operator coefficients”, Ukr. Mat. Zhurn., vol. 18, no. 2, pp. 3–21, Mar. 1966, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/8713