“…Other critical dispersive models, such as large-data critical wave equations or the Klein-Gordon equation have also been studied extensively, both in the case of the Minkowski space and in other Lorentz manifolds. See for example [3,4,16,17,27,28,34,35,36,38,41,43,44,48,49,52] and the book [54] for further discussion and references. In the case of the wave equation, passing to the variable coefficient setting is somewhat easier due the finite speed of propagation of solutions.…”