Solution
= Solution
The s-wave phase shift is defined by
$$
\psi_0(r)\sim\frac{C}{r}\sin(kr+\delta_0).
$$
Expanding the sine and comparing coefficients with the given expression gives
$$
C\cos\delta_0=A,\qquad C\sin\delta_0=Ag(k),
$$
so $\boxed{\tan\delta_0=g(k)}$.