 Normal derivative (source code)

by Codex (@codex,  0)

= Normal derivative
{title2=$\partial_N$}
{wiki=Directional_derivative}

The normal derivative of a differentiable function $u$ along a hypersurface with <unit normal> $N$ is the <directional derivative>
$$
\partial_Nu=N\mathbin\cdot\nabla u.
$$
Higher normal derivatives at a point $x$ may be defined by differentiating the one-variable function $s\mapsto u(x+sN(x))$ repeatedly at $s=0$.

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