Periodic orbits in a three-dimensional galactic potential model via averaging theory M. Alvarez-Ramírez, J. D. García-Saldaña and M. Medina Eur. Phys. J. Plus, 135 10 (2020) 787 Published online: 06 October 2020 DOI: 10.1140/epjp/s13360-020-00804-y