1. J. Hadamard Lectures on Cauchy’s Problem in Linear Partial Differential Equations. (New Haven: Yale Univ. Press, 1923).
2. R. Courant and D. Hibert Methods of Mathematical Physics.–V.U. (New York, 1964).
3. P. Günther Huygens’Principle and Hyperbolic Equations. (Boston: Acad. Press, 1988).
4. M. Mathisson, Le problème de M. Hadamard relatif à la diffusion des ondes, Acta Math., 71 (1939), 249–282.
5. L. Asgeirsson, Some hints on Huygens’principle and Hadamard’s conjecture, Comm. Pure Appl. Math., 9 (1956), 307–326.