  The Tricomi equation is a second-order partial differential equation of mixed
elliptic-hyperbolic type. It was first analyzed in the work by Francesco
Giacomo Tricomi (1923) on the well-posedness of a boundary value problem. The
Tricomi equation can be transformed into the corresponding elliptic or
hyperbolic Euler-Poisson-Darboux equation, and has a close connection with
transonic flow and isometric embedding. It has different degeneracy from a
closely related equation, the Keldysh equation.
