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Revision difference : Clairaut's theorem
Version 4 Version 3
\paragraph{Theorem.}If $\mathbf{F}:\mathbb{R}^n \to \mathbb{R}^m$ is a function whose second derivatives exist and are continuous on a set $S \subseteq \mathbb{R}^n$, then If $$\mathbf{F}:\mathbb{R}^n \to \mathbb{R}^m$$ is a function whose second derivatives exist and are continuous on a set $S \subseteq \mathbb{R}^n$, then
$$ \frac{\partial^2 f}{\partial x_i \partial x_j}=\frac{\partial^2 f}{\partial x_j \partial x_i} $$ on $S$.\\ $$ \frac{\partial^2 f}{\partial x_i \partial x_j}=\frac{\partial^2 f}{\partial x_j \partial x_i}. $$
This is commonly referred to as simply 'the equality of mixed partials'. It is usually first presented in a vector calculus course, and is useful in this context for proving basic properties of the interrelations of gradient, divergence, and curl. I.e., if $\mathbf{F}$ is a function satisfying the hypothesis, then $\nabla \cdot (\nabla \times \mathbf{F}) =0$ and $\nabla \times \nabla \mathbf{F} = \mathbf{0}$. This is commonly referred to as simply 'the equality of mixed partials'. It is usually first presented in a vector calculus course, and is useful in this context for proving basic properties of the inter-relations of gradient, divergence, and curl. I.e., if $\mathbf{F}$ is a function satisfying the hypothesis, then $$\nabla \cdot (\nabla \times \mathbf{F}) =0$$ and $$\nabla \times \nabla \mathbf{F} = \mathbf{0}$$.