Critical strength of attractive central potentials

We obtain several sequences of necessary and sufficient conditions for the existence of bound states applicable to attractive (purely negative) central potentials. These conditions yield several sequences of upper and lower limits on the critical value, g^(ell)_c, of the coupling constant (strength), g, of the potential, V(r) = -gv(r), for which a first ell-wave bound state appears, which converges to the exact critical value.