1. S. Agmon, ``Boundary value problems for equations of mixed type'' in Convegno Internazionale sulle Equazioni Lineari alle Derivate Parziali (Trieste, 1954), Edizioni Cremonese, Rome, 1955, 54--68.
2. J. Barros-Neto and I. M. Gelfand, Fundamental solutions for the Tricomi operator, Duke Math. J. 98 (1999), 465--483.
3. R. Courant and D. Hilbert, Methods of Mathematical Physics, Vol. I, Interscience, New York, 1953.
4. G. Darboux, Leçons sur la théorie générale des surfaces, Vol. II, 2d ed., reprint, Grands Class. Gauthier-Villars, Éd. Jacques Gabay, Sceaux, France, 1993.
5. A. Erdélyi, W. Magnus, F. Oberhettinger, and F. G. Tricomi, Higher Transcendental Functions, Vols. I, II, McGraw-Hill, New York, 1953, ; Vol. III, 1955.