PX275 - C2a - grad squared

\begin{array}{r} \vec{a} = \vec{\nabla} ϕ ⟹ \vec{\nabla} \cdot a = \vec{\nabla} \cdot (\vec{\nabla} ϕ) = \nabla^{2} ϕ \end{array}

\nabla^{2} ϕ = (\frac{\partial^{2} ϕ}{\partial x^{2}} + \frac{\partial^{2} ϕ}{\partial y^{2}} + \frac{\partial^{2} ϕ}{\partial z^{2}})