can you explain this step further: 4. **Change of Variables**: Assume a solution of the form \(\phi(r) = \frac{u(r)}{r}\). Then: \[ \frac{d\phi}{dr} = \frac{1}{r} \frac{du}{dr} - \frac{u}{r^2} \] \[ \frac{d}{dr} \left( r^2 \frac{d\phi}{dr} ight) = \frac{d}{dr} \left( r \frac{du}{dr} - u ight) = r \frac{d^2u}{dr^2} \]