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From: Roy Stogner <roystgnr@ic...>  20060729 08:54:52

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 Gradient/RealGradient.  Roy 