M. Z. Islam, M. S. Alam and M. B. Hossain, A unified Krylov-Bogoliubov-Mitropolskii method for solving hyperbolic-type nonlinear partial differential systems, Vietnam J. Mech. 30 (2008) 11–19. DOI: https://doi.org/10.15625/0866-7136/30/1/5607.