Conditional quasi-exact solvability of the quantum planar pendulum and of its anti-isospectral hyperbolic counterpartSimon Becker, Marjan Mirahmadi, Burkhard Schmidt, Konrad Schatz and Bretislav FriedrichEur. Phys. J. D, 71 6 (2017) 149DOI: https://doi.org/10.1140/epjd/e2017-80134-6