  We prove two uniqueness theorems for solutions of linear and nonlinear wave
equations; the first theorem is in the Minkowski space while the second is in
the domain of outer communication of a Kerr black hole. Both theorems concern
ill posed Cauchy problems on smooth, bifurcate, characteristic hypersurfaces.
In the case of the Kerr space-time this hypersurface is the event horizon of
the black hole.
