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2. Prandtl, L.: über Flüssigkeitsbewegung bei sehr kleiner Reibung. Verhandlungen des III. Internationalen Mathematiker Kongresses, Heidelberg, 8–13 August 1904. B. G. Teubner, Leipzig, pp. 485–491 (1905). (in German)
3. Tikhonov, A.N.: On systems of differential equations containing parameters. Matematicheskii Sbornik 27(69), 1, 147–156 (1950). (in Russian)
4. Tikhonov, A.N.: Systems of differential equations containing small parameters in the derivatives. Matematicheskii Sbornik 31(73), 3, 575–586 (1952). (in Russian)
5. Vasil’eva, A.B., Butuzov, V.F.: Asymptotic Expansions of Solutions of Singularly Perturbed Equations. Nauka, Moscow (1973). (in Russian)