  We consider the Cauchy problem for a second order quasi-linear partial
differential equation with an admissible parabolic degeneration such that the
given functions described the initial conditions are defined on a closed
interval. We study also a variant of the inverse problem of the Cauchy problem
and prove that the considered inverse problem has a solution under certain
regularity condition. We illustrate the Cauchy and the inverse problems in some
interesting examples such that the families of the characteristic curves have
either common envelopes or singular points. In these cases the definition
domain of the solution of the differential equation contains a gap.
