1. Leçons sur la theorie generale des surfaces et les applications geometriques du calcul infinitesimal;Darboux,1896
2. Group analysis of differential equations;Ovsyannikov,1978
3. The variational bicomplex for second order scalar partial differential equations in the plane;Anderson,1994
4. What is integrability?, Springer series in nonlinear dynamics;Calogero,1991
5. Applications of the Laplace transformation to the classification of integrable quasilinear hyperbolic equations;Zhiber,1995