  Reflection of a shock from a solid wedge is a classical problem in gas
dynamics. Depending on the parameters either a regular or a irregular
(Mach-type) reflection results. We construct regular reflection as an exact
self-similar solution for potential flow. For some upstream Mach numbers and
isentropic coefficients, a solution exists for all wedge angles allowed by the
sonic criterion. This demonstrates that, at least for potential flow, weaker
criteria are false.
