L. V. Fardigola, “Transformation operators in controllability problems for the degenerate wave equation with variable coefficients”, Ukr. Mat. Zhurn., vol. 70, no. 8, pp. 1128–1142, Aug. 2018, Accessed: Feb. 16, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/1623