1. A. A. Dorodnitsyn, “Asymptotic Solution of the Van der Pol Equation,” Prikl. Mat. Mekh. 11, 313–328 (1947).
2. A. B. Vasil’eva and V. F. Butuzov, Asymptotic Methods in the Theory of Singular Perturbations (Vysshaya Shkola, Moscow, 1990) [in Russian].
3. C. V. Pao, Nonlinear Parabolic and Elliptic Equations (Plenum, New York, 1992).