= Conormal flux of a divergence-form operator
{title2=$\sum_{i,j}a_{ij}D_i u\,\nu_j$}
For sufficiently regular coefficients and a <function> $u$, this <boundary> quantity is the outward flux appearing when a divergence-form principal operator is integrated by parts. Its zero condition is the natural <Neumann boundary condition> for the corresponding <bilinear form>. With only <bounded> coefficients and a Sobolev solution, the <weak formulation> can encode that natural condition without asserting that a <pointwise> <boundary> <derivative> exists.
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