1. A. B. Vasil’eva and V. F. Butuzov, AsymptoticMethods in the Theory of Singular Perturbations (Vysshaya Shkola, Moscow, 1990) [in Russian].
2. A. M. Il’in, Matching of Asymptotic Expansions of the Solutions of Boundary-Value Problems (Nauka, Moscow, 1989) [in Russian].
3. V. F. Butuzov, “On periodic solutions to singularly perturbed parabolic problems in the case of multiple roots of the degenerate equation,” Zh. Vychisl.Mat. i Mat. Fiz. 51(1), 44–55 (2011) [Comput. Math. and Math. Phys. 51 (1), 40–50 (2011)].
4. C. V. Pao, Nonlinear Parabolic and Elliptic Equations (Plenum Press, New York, 1992).