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=== Mixed Formulation for Poisson Equation ===
In this example the poisson equation is solved with a mixed approach: the stable FE space obtained using Brezzi-Douglas-Marini polynomial of order 1 and Dicontinuos element of order 0 is used.
<math>-\mathrm{div}\ ( \mathbf{\sigma} (x, y) ) ) = f (x, y) \qquad \mbox{ in } \Omega</math>
<math> \sigma (x, y) = \nabla u (x, y) \qquad \mbox{ in } \Omega</math>
<math>u(x, y) = 0 \qquad \mbox{ on } \Gamma_D</math>
<math>(\sigma (x, y) ) \cdot \mathbf{n} = \sin (5x) \qquad \mbox{ on } \Gamma_N</math>
A complete description of the problem is avilable on the [http://fenicsproject.org/documentation/dolfin/1.2.0/python/demo/pde/mixed-poisson/python/documentation.html Fenics website.]
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