In this question you are requested to calculate the phase shifts in scattering on a 3D delta function \( V(r)=u\delta(x)\delta(y)\delta(z) \) . For this puropose think of it as a small sphere of radius \( a \) and potential \( U_0 \) such that \( u = (4pi/3) U_0 a^3 \) . Later take the limit \( a \rightarrow 0 \) .