S. Lie: Math. Ann.32 (1888) 213.
S. Lie:Theorie der Transformationgruppen, B.G. Teubner, Leipzig, 1888, 1890, 1893.
P.J. Olver:Applications of Lie Groups to Differential Equations, Springer, New York, 1993.
D. Levi and P. Winternitz: Phys. Lett. A152 (1991) 335.
D. Levi and P. Winternitz: J. Math. Phys.34 (1993) 3713.
D. Levi and P. Winternitz: J. Math. Phys.37 (1996) 5551.
R. Hernandez Heredero, D. Levi, M.A. Rodriguez, and P. Winternitz: J. Phys. A: Math. Gen.33 (2000) 5025.
R. Floreanini and L. Vinet: J. Math. Phys.36 (1995) 7024.
G.R.W. Quispel, H.W. Capel, and R. Sahadevan: Phys. Lett. A170 (1992) 379.
V.A. Dorodnitsyn: inVINITI Acad. Sci. USSR, Itogi Nauki i Techniki,34 (1989) 149 (in Russian); see English translation in J. Sov. Math.55 (1991) 1490.
V.A. Dorodnitsyn: Int. J. Mod. Phys. C (Phys. Comp.)5 (1994) 723.
V. Dorodnitsyn:Group Properties of Difference Equations, Max Press, Moscow, 2000 (in Russian).
V. Dorodnitsyn, R. Kozlov, and P. Winternitz: J. Math. Phys.41 (2000) 480.
D. Levi, S. Tremblay, and P. Winternitz: J. Phys. A: Math. Gen.33 (2000) 8507.