Solution
= Solution
Irrotational means $\nabla\times E=0$. For $E=\nabla f$, its $i$th curl component is $\varepsilon_{ijk}\partial_j\partial_kf=0$ because mixed partials are symmetric.
= Solution
Irrotational means $\nabla\times E=0$. For $E=\nabla f$, its $i$th curl component is $\varepsilon_{ijk}\partial_j\partial_kf=0$ because mixed partials are symmetric.