1. Blanco, R.A., Manno, G., Pugliese, F.: Contact relative differential invariants for non generic parabolic Monge-Ampère equations. Acta Appl. Math. 101(1–3), 5–19 (2008)
2. Cotton, E.: Sur les invariants différentiels de quelques équations linearies aux dérivées partielles du second ordre. Ann. Sci. Ec. Norm. Super 17, 211–244 (1900)
3. Euler, L.: Calculi Integralis, vol. 3. Impenfis Academiac Imperialis Scientiarium, Petropoli (1770)
4. Ibragimov, N.H.: Group classification of second order differential equations. Dokl. Akad. Nauk SSSR 183(2), 274–277 (1968) (in Russian). English translation in Sov. Math. Dokl. 9(6), 1365–1369 (1968)
5. Ibragimov, N.H.: Equivalence groups and invariants of linear and non-linear equations. Archives of ALGA. Blekinge Institute of Thechnology, Karlskrona, Sweden, vol. 1, pp. 9–65 (2004)