Principal diffusion coefficients
= Principal diffusion coefficients
{title2=$c(s),\quad c(s)+sc\prime(s)$}
For flux $c(|\nabla u|)\nabla u$, the <derivative> matrix is $c(s)I+c'(s)\nabla u\otimes\nabla u/s$ with $s=|\nabla u|>0$. Its tangential and normal coefficients are $c(s)$ and $c(s)+sc'(s)$. Positivity of the scalar conductance alone therefore does not guarantee forward parabolicity.