1. Agmon, S., Problèmes mixtes pour les équations hyperboliques d'ordre supéreure. Colloques Internationales du Centre National de la Récherche Scientifique, 117, Paris 1962.
2. Agemi, R., On energy inequalities of mixed problems for hyperbolic equations of second order. J. Fac. Science, Hokkaido Univ. (1) 21, 221–236 (1971).
3. Agranovich, M. S., Theorem on matrices depending on parameters and its applications to hyperbolic systems. Funktstional'nyi Analiz i ego Prilozheniya 6, 1–11 (1972).
4. Bloom, C. O., On the validity of the geometrical theory of diffraction by starshaped cylinders. J. Math. Anal. and Appl. 40, 107–121 (1972).
5. Borovikov, V. A., The elementary solution of partial differential equations with constant coefficients. Trudy Moskov. Mat. Obšč. 68, 159–257 (1959).