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A nodal curl-free mode outside the compatible gradient image
Example
On one tetrahedron, compare the naive vector nodal space with the compatible scalar-to-edge gradient map of Lowest-order Whitney and Nedelec de Rham spaces. The vector field lies in the nodal vector space, has zero curl, and is not the gradient of a continuous piecewise-affine scalar. Thus the naive nodal curl-curl matrix has a zero-energy mode outside that compatible gradient image. This identifies a discrete kernel mismatch; it does not claim that gradients are absent from the continuum curl kernel.
Facts & Assumptions
Given: Countable Choice for the real gauge convention; a reference tetrahedron, the scalar nodal space and compatible edge space of Lowest-order Whitney and Nedelec de Rham spaces, and the gauge formulation of Kernel quotient and gauge for discrete curl-curl.
Countable Choice is assumed by the real product and the stable gauge supplier; no full Axiom of Choice is used. (The Axiom of Countable Choice ())
The scalar nodal space consists of affine functions on the tetrahedron, so every member has constant gradient. (Lowest-order Whitney and Nedelec de Rham spaces)
The pure curl-curl energy vanishes on its curl kernel, and the mixed gauge formulation fixes a representative by orthogonality to that kernel. (Kernel quotient and gauge for discrete curl-curl, Stable mixed curl-curl formulation with kernel gauge)
Verification
The field is vector-valued affine, so it belongs to . Its componentwise curl is zero because its only nonzero component depends only on . Hence .
By [F1], every has constant gradient, while is nonconstant on the interior of . Thus . The kernel of the naive nodal curl-curl energy is therefore strictly larger than the compatible discrete gradient image, and its representative is not unique without a gauge as in [F2].
Depends on
Used by
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Dependency tree · two levels
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Sources
- Long Chen, Finite Element Methods for Maxwell Equations, complete lecture notes (standard reference, not scraped)