“…The best known of them are the quadratically superintegrable systems, i.e., those allowing two (at most) second-order integrals of motion. Their study began in the mid 1960s [56] and by now they have been completely classified in conformally flat spaces [57,58,59,60,61,62,63,64,65,66]. In the higher-order case, the direct approach for determining integrals of motion becomes more and more difficult as their order increases, as it has recently been shown for third [67,68,69,70] and quartic order [71].…”