On Fri, 28 Jul 2006, David Xu wrote:
> I was wondering how to write down an equation containing mixed derivative in
> libMesh's element matrix (Ke) building step?
> For exmaple, Uxx+Uyy+Uxy (U is 2nd order derivative)
If you need mixed second derivatives of a shape function, take a look
at example 15, which solves a biharmonic equation by a direct Galerkin
method which computes laplacians at each quadrature point.
If you need derivatives of your solution, take a look at how solutions
and their derivatives are accumulated in example 13 - you'd do the
same thing but with Tensor/RealTensor classes instead of