1. Arnold, V. I., Chapitres supplémentaires de la théorie des équations différentielles ordinaires, Moscow: Mir, 1980.
2. Belitskii, G. R., Invariant Normal Forms of Formal Series, Funct. Anal. Appl., 1979, vol. 13, no. 1, pp. 46–47; see also: Funktsional. Anal. i Prilozhen., 1979, vol. 13, no. 1, pp. 59–60.
3. Belitskii, G. R., Normal Forms in Relation to the Filtering Action of a Group, Trans. Moscow Math. Soc., 1981, no. 2, pp. 1–39; see also: Tr. Mosk. Mat. Obs., 1979, vol. 40, pp. 3–46.
4. Bourbaki, N., Groupes et algèbres de Lie: Chapitres 7 et 8, Paris: Hermann, 1975.
5. Bruno, A.D., Analytic Form of Differential Equations: 1, Tr. Mosk. Mat. Obs., 1971, vol. 25, pp. 119–262 (Russian); see also: Trans. Moscow Math. Soc., 1971, vol. 25, pp. 131–288; Bruno, A.D., Analytic Form of Differential Equations: 2, Tr. Mosk. Mat. Obs., 1972, vol. 26, pp. 199–239 (Russian); see also: Trans. Moscow Math. Soc., 1972, vol. 26, pp. 199–239.