Izvekov IG, Martynenko EV. On Gevrais classes of certain self-adjoint differential operators with degeneration. Ukr. Mat. Zhurn. [Internet]. 1993Dec.25 [cited 2024Nov.24];45(12):1622–1626. Available from: https://umj.imath.kiev.ua/index.php/umj/article/view/5969