Clairaut's Theorem

Theorem
Let $\map f {x, y}$ be a function of the two independent variables $x$ and $y$.

Let $\map f {x, y}$ be continuous.

Let the partial deriviatives of $f$ also be continuous.

Then:
 * $\dfrac {\partial^2 f} {\partial x \partial y} = \dfrac {\partial^2 f} {\partial y \partial x}$