Gradient for Functions of the Radius
For a function fdefined in terms of spherical radial coordinate
we can write
In spherical polar coordinates
\[f\]
is not a function of \[\theta\]
or \[\phi\]
so 
Example: If
then
Example: If
then \[\mathbf{\nabla} f = - \frac{1}{r^2} \mathbf{e}_r\]
