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Add feec docs#380

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Add feec docs#380
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dtoshniwal marked this pull request as ready for review June 2, 2026 11:04
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dtoshniwal requested review from J15525 and dccabanas June 2, 2026 11:05
* $l(v) = \int_\Omega f v \, dx$ is a linear form.

This formulation requires $u$ and $v$ to only have (square-integrable) first derivatives and thus describes a much larger class of situations than the strong form.
For instance, the graph of the solution for a discontinuous choice of $f$ on a square domain is shown below:

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Seems like there is a figure missing?

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This should be removed


Such a discretization fails on the L-shaped domain: the discrete solution lives in a proper closed subspace of ``H(\text{curl}; \Omega) \cap \mathring{H}(\text{div}; \Omega)``, producing spurious modes that persist under mesh refinement.

![Spurious solution from the primal formulation on the L-shaped domain](../../assets/Theory/LshapeWrong-1.png)

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The figure should be a tad smaller.

The spaces ``V_h^0`` and ``V_h^1`` need to be chosen in a "compatible" manner (in the sense described in the following sections), but when they are, the discrete solution converges to the true solution, and we avoid the issues associated with the primal formulation.
Note that this is true even when ``V_h^1`` is an appropriately chosen ``H^1``-conforming space (e.g., that of ``C^1`` smooth B-spline edge elements): the key difference from the primal formulation is that now the ``H(\text{curl};\Omega)``-norm restricted to such spaces is not equivalent to the ``H^1`` norm.

![Correct solution from the mixed FEEC formulation on the L-shaped domain](../../assets/Theory/LshapeFEEC-1.png)

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Same as above.

@J15525 J15525 mentioned this pull request Jun 16, 2026
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@dccabanas
dccabanas force-pushed the add-feec-docs branch 2 times, most recently from 7a336c5 to c6dccfa Compare June 25, 2026 06:27
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