R. K. C. and S. Alam M., Effect of higher approximation of Krylov-Bogoliubov-Mitropolskii’s solution and matched asymptotic solution of a differential system with slowly varying coefficients and damping near to a turning point, Vietnam J. Mech. 26 (2004) 182–192. DOI: https://doi.org/10.15625/0866-7136/26/3/5702.