Focus point: cancer and HIV/AIDS dynamics—from optimality to modelling Amar Debbouche, Juan J. Nieto and Delfim F. M. Torres Eur. Phys. J. Plus, 136 2 (2021) 165 Published online: 02 February 2021 DOI: 10.1140/epjp/s13360-021-01154-z