Geodesic
Perturbation methods in group analysis
Itogi Nauki i Tekhniki. Seriya Sovremennye Problemy Matematiki. Noveishie Dostizheniya, Tome 34 (1989), pp. 85-147.

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In this paper we describe the theory of approximate groups of transformations which lets us constructively find approximate symmetries of equations with a small parameter. For evolution equations we introduce the concept of formal symmetries and Bдcklund transformations. The theory is applied to a large class of wave and evolution equations. 
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     author = {V. A. Baikov and R. K. Gazizov and N. Kh. Ibragimov},
     title = {Perturbation methods in group analysis},
     journal = {Itogi Nauki i Tekhniki. Seriya Sovremennye Problemy Matematiki. Noveishie Dostizheniya},
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     volume = {34},
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V. A. Baikov; R. K. Gazizov; N. Kh. Ibragimov. Perturbation methods in group analysis. Itogi Nauki i Tekhniki. Seriya Sovremennye Problemy Matematiki. Noveishie Dostizheniya, Tome 34 (1989), pp. 85-147. http://geodesic.mathdoc.fr/item/INTD_1989_34_a1/