We consider the reflection of an incident shock by a wedge for the “nonlinear wave system.” For wedge angle sufficiently small, it is shown that a homotopy between weak regular reflection and Mach or irregular reflection, parameterized by shock strength, does not exist.

This suggests that Mach reflection is not only irregular, but also illusory, and that attempts to compute solution structure in the neighborhood of the “triple point” will produce results necessarily dependent on the numerical dissipation.