how to expand grad f * (p-p_0) in spherical polar coordinates
in spherical polar coordinates:
\nabla f = \frac{\partial f}{\partial r} e_r+ \frac{1}{r sin\theta}\frac{\partial f}{\partial \phi} e_{\phi}+ \frac{1}{r}\frac{\partial f}{\partial \theta} e_{\theta}
p=(r,\phi,\theta) and...