Stokes equation
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Stokes\' equation is a good place to see the formulation and the assembly of various blocks in the system.
As in the Poisson sample, it is optionnally solved as a non-linear problem.

The finite element type for the hydrostatic pressure P is given by *degreeP*. When positive, it uses a discontinuous finite element
P0 or P1D. When negative, it uses a lagrange finite element with a degree equal to degreeU+degreeP. Hence *degreeU=2* and *degreeP=-1* means lagrange degree 2 for U and lagrange degree 1 for P.

.. highlight:: c++

.. literalinclude:: ./nvi_Stokes_0.cpp

Results
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.. figure:: Stokes-cavity.png
    :align: center

Log file
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.. literalinclude:: ./Stokes_0_grid2.log
    :language: text