Radial derivative
= Radial derivative
{title2=$\partial_r u(x)=\nabla u(x)\cdot x/|x|$}
Away from the origin, the radial derivative is the <directional derivative> along the outward radial unit vector. For a <Sobolev space> function it is the same scalar product with the <weak gradient>, defined <almost everywhere>. On an <annulus>, it has no singular direction at the origin.