1. Leifur Asgeirsson, Some hints on Huygens’ principle and Hadamard’s conjecture,Comm. Pure Appl. Math. 9 (1956), 307–326.
2. Luigi Bianchi,Vorlesungen über Differentialgeometrie, Leipzig: B. G. Teubner-Verlag (1899).
3. J. Carminati and R. G. McLenaghan, An explicit determination of the space-times on which the conformally invariant scalar wave equation satisfies Huygens’ principle,Ann. Inst. Henri Poincaré, Phys. théor. 44 (1986), 115–153; Part II, 47 (1987), 337-354; Part III, 48 (1988), 77-96.
4. Richard Courant and David Hubert,Methoden der Mathematischen Physik, Band 2, Berlin: Springer-Verlag (1937).
5. F. G. Friedlander,The wave equation on a curved space-time, Cambridge: Cambridge Univ. Press (1975).