Sextic anharmonic oscillators and Heun differential equations Primitivo B. Acosta-Humánez, Mourad E. H. Ismail and Nasser Saad Eur. Phys. J. Plus, 137 7 (2022) 835 Published online: 20 July 2022 DOI: 10.1140/epjp/s13360-022-03029-3