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A repeated unit-cube Maxwell eigenvalue cluster
Example
On the unit cube with constant , the two fields are independent PEC eigenfields with the same eigenvalue . Thus the continuous eigenspace at this value has multiplicity at least two, and a convergent edge discretization must reproduce the full cluster. The theorem asserts convergence of eigenspaces, not a canonical choice of one eigenvector.
Facts & Assumptions
Given: Countable Choice, the Axiom of Choice and Dependent Choice; the unit cube; constant ; the PEC curl-curl formulation; and a shape-regular refining family of matching affine tetrahedral meshes with . Assume the three supplier statements on regular decomposition (Regular decomposition of zero-tangential H(curl) fields), discrete compactness (Discrete compactness and uniform Poincare inequality for Nedelec fields) and uniformly bounded commuting cochain projections (Uniformly bounded commuting projection for lowest-order Nedelec spaces) hold for this exact domain, PEC boundary convention and mesh family.
The PEC eigenrelation is for every gauge test field. (PEC Maxwell cavity eigenproblem and full-kernel gauge)
The edge spectral theorem approximates every isolated positive eigenvalue with full multiplicity and converges the associated eigenspaces in gap. (Spectral convergence and no nonzero pollution for PEC edge elements)
The compact coercive solution operator has isolated positive eigenvalues of finite multiplicity. (Discrete spectrum and min–max principle for a compact coercive form)
The choice assumptions are hypotheses of the edge spectral theorem and its compact spectral suppliers. (The Axiom of Choice, The Axiom of Countable Choice (), The axiom of dependent choice: a relation in which every element is related to something admits an -indexed chain)
is the graph-norm closure of . (Zero tangential trace H(curl) space)
Verification
For positive integers , set . It is divergence-free and its tangential component vanishes on and because its third component vanishes there; on it is normal. A product cutoff vanishing within distance of the cube faces, equal to one outside the boundary layer, and with derivatives gives compactly supported smooth approximants . The -derivative term crossed with is zero; on and layers the corresponding sine factor is , so . The remaining curl error is bounded on a layer of volume , hence tends to zero in . Also is bounded and supported in a layer of volume , so it tends to zero in . Therefore .
Direct differentiation gives . Testing against compactly supported smooth fields and integrating by parts proves the weak eigenrelation; by the closure definition [F5] and continuity of both pairings it holds for every PEC test field. Testing with each in the curl kernel gives , so the fields lie in the full-kernel gauge.
The eigenvalue is positive and isolated by [F3].
For and the eigenvalue is . Their mass inner product is zero because ; hence they are linearly independent and the eigenspace multiplicity is at least two.
Apply [F2] to the full eigenspace cluster: exactly its actual multiplicity of discrete eigenvalues converges to this value and the discrete eigenspaces converge in gap. The explicit two orthogonal modes show the multiplicity is at least two; no individual basis vector is selected by the theorem.
Depends on
- PEC Maxwell cavity eigenproblem and full-kernel gauge
- Zero tangential trace H(curl) space
- Spectral convergence and no nonzero pollution for PEC edge elements
- Discrete spectrum and min–max principle for a compact coercive form
- Eigenvalues, eigenvectors, eigenspaces $E_\lambda(T)=\ker(T-\lambda I)$, and the spectrum $\sigma_F(T)$ of an endomorphism
- The Axiom of Choice
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
- The axiom of dependent choice: a relation in which every element is related to something admits an $\mathbb{N}$-indexed chain
- Regular decomposition of zero-tangential H(curl) fields
- Discrete compactness and uniform Poincare inequality for Nedelec fields
- Uniformly bounded commuting projection for lowest-order Nedelec spaces
Used by
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