The Mixed Bitsadze-Lavrent'ev-Tricomi Boundary Value Froblem

It is well known that F.G. Tricomi (1923) is the originator of the theory of boundary value problems for mixed type equations by establishing the Thicomi equation: y. u_(xx) u_(yy)=0 which is hyperbolic for y<0, elliptic for y(?)0, and parabolic for y=0 and then applied it in the theory of transonic flows.