“…There are several related toy models which also have continuous spectrum, see for example [20]. It is only when the supermembrane is restricted by certain topological conditions, non-trivial central charges, that the spectrum becomes discrete, with eigenvalues accumulating at infinity [23,24,25,26,27]. In order to analyse with more precision the supermembrane and super 5-brane potentials, and even more complicated potentials as in the BLG and ABJM theories it is very useful to consider a necessary and sufficient condition to have a discrete spectrum.…”