Consider scattering by a δ-function shell potential: V(r)=αδ(r−a).
(a) Find the phase shifts δl for all l.
(b) Find the total cross section in the low-energy limit (ka≪1), keeping only l=0.
(c) Show that when 2mkaα/ℏ2→−1 there is a resonance in the s-wave.