Nonlinear Klein–Gordon equation and the Bose–Einstein condensation E. Megías, M. J. Teixeira, V. S. Timoteo and A. Deppman Eur. Phys. J. Plus, 137 3 (2022) 325 Published online: 09 March 2022 DOI: 10.1140/epjp/s13360-022-02511-2