2 citations to https://www.mathnet.ru/eng/dan33269

1. Yu. K. Sabitova, “On location of the spectrum of the Tricomi problem”, Siberian Math. J., 56:1 (2015), 160–176            
2. N. A. Lar'kin, “On solvability of stationary transonic equations in the unbounded domain”, Math. USSR-Sb., 70:1 (1991), 31–45