As was pointed out by Ma,l) when we want to determine the energy levels of the bound states from the poles in the positive imaginary axis of the complex k-plane (k= I k I, k is momentem), there may appear the redundant poles having no connection with the bound states. He discussed the motion of a particle in the potential V(r) = -Vo exp(-Iff).(1)The S-matrix for the S-wave is
S(k)= J;.(a)r(ip-}--1) (a/z)-2ip, (Z)
J-;.(a)r( -zp+ 1)where p=2k/p., a=2../2mVo/p., m is the mass of the considered particle. Therefore, S(k) has poles in the positive imaginary axis, when and J-;.(a)=O r(ip+1)=oo.(3)The condition (3) gives the correct results for the bound states, but (4) the redundant poles. Namely the redundant poles appear at