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Adaptive finite-element methods — Examples

1 · Prerequisites

2 · Summary

These finite checks demonstrate the exact descending-prefix bulk rule, P0 data-oscillation projection on a reference triangle, nonconstant conormal-flux oscillation across a coefficient interface, and failure of the stated sufficient small-mesh inequality for a prescribed convection constant. Each verification uses the definitions and hypotheses on adaptive-finite-element-methods. The coefficient and convection examples do not assert general convergence or failure of convergence.

3 · Logical flowchart

4 · Definitions, theorems and proofs

None yet.

5 · Examples, counterexamples and false statements

ExampleConstruction: AI-generatedVerification: AI-adaptedprecheck passjudge pass (gpt-6-luna)Open item page →

A coefficient jump creates nonzero edge-flux oscillation

Example

Assume Countable Choice for the cited Sobolev/L2 interface.

Let E={(0,y):0≤y≤1} be shared by K−=conv⁡{(−1,0),(0,0),(0,1)} and K+=conv⁡{(1,0),(0,0),(0,1)}. Set A−=1, A+=1+y, and U(x,y)=x. On each triangle A is positive and Lipschitz; its traces differ on E.

Facts & Assumptions

[A1]

Countable Choice is the background assumption in the cited Sobolev and L2 suppliers (The Axiom of Countable Choice (ACω)).

Given: Countable Choice, The piecewise-Lipschitz coefficient interface and projected edge flux oscillation of Residual indicators and data oscillation for P1 AFEM.

[F1]

For a scalar diffusion coefficient, the outward conormal flux on a side is A∇U⋅n; edge oscillation is the squared L2 distance of the outward flux sum from its edge mean (Residual indicators and data oscillation for P1 AFEM).

Verification

1.1A1F1givenalgebra

The gradient is ∇U=(1,0). The outward normals on E are n−=(1,0) and n+=(−1,0), so the sum of outward conormal fluxes is JE(y)=1−(1+y)=−y. Its edge mean on [0,1] is −1/2.

2.1step 1.1algebra∎

Since hE=1, hE∥JE−ΠE0JE∥L2(E)2=∫01(y−12)2 dy=112>0. Thus the coefficient interface creates a nonzero projected edge contribution. No general reliability or complexity assertion for this coefficient model follows from the example.

Finite check

The edge integral is the elementary variance of y on [0,1], equal to 1/12.

ExampleConstruction: AI-generatedVerification: AI-adaptedprecheck passjudge pass (gpt-6-luna)Open item page →

A coarse mesh can miss the convection smallness condition

Example

Assume Countable Choice for the cited Sobolev/L2 interface.

Fix a positive mesh diameter h0, exponent s>0, and a stipulated positive dual-pickup constant CD. The sufficient small-mesh parameter in Quasi-orthogonality for constant convection on a fine initial mesh is μ=CDh0s∣b∣.

Facts & Assumptions

[A1]

Countable Choice is the background assumption in the cited Sobolev and L2 suppliers (The Axiom of Countable Choice (ACω)).

Given: Countable Choice, A prescribed mesh scale h0>0, s>0, CD>0, and a constant divergence-free vector b∈R2.

[F1]

The nonsymmetric quasi-orthogonality estimate assumes μ=CDh0s∣b∣<1 (Quasi-orthogonality for constant convection on a fine initial mesh).

Verification

1.1A1F1givenconstructalgebra

Choose any unit vector e and set b=2(CDh0s)−1e. This is constant and divergence-free, and direct substitution gives μ=2.

2.1step 1.1algebra∎

The strict sufficient condition μ<1 therefore fails on this prescribed mesh. This only shows that the stated sufficient estimate cannot be invoked from its hypotheses; it does not imply failure of convergence or of the PDE solution.

Finite check

The substitution is exact: CDh0s⋅2(CDh0s)−1=2.

ExampleConstruction: AI-generatedVerification: AI-adaptedprecheck passjudge pass (gpt-6-luna)Open item page →

A deterministic Dörfler mark on three cells

Example

Take squared cell indicators (9,4,1) in the stated identifier order and θE=3/5.

Facts & Assumptions

Given: Three nonnegative squared indicators and the deterministic prefix rule of Dorfler marking and conforming newest-vertex refinement.

[F1]

Dörfler marking selects the shortest descending-value prefix whose sum reaches the declared fraction of the total (Dorfler marking and conforming newest-vertex refinement).

Verification

1.1F1givenalgebra

The total is 9+4+1=14, so the threshold is (3/5)14=42/5=8.4. The sorted descending prefix sums are 9,13,14.

2.1step 1.1algebra∎

The first prefix already has sum 9≥8.4, while the empty prefix has sum 0<8.4. Thus the unique minimal mark is the first cell, regardless of the later tie rule.

Finite check

The three prefixes are exactly ∅, {1}, {1,2} and {1,2,3} with sums 0,9,13,14; the first threshold-crossing prefix has cardinality one.

ExampleConstruction: AI-generatedVerification: AI-adaptedprecheck passjudge pass (gpt-6-luna)Open item page →

Projected data oscillation on a reference triangle

Example

Assume Countable Choice for the cited Sobolev/L2 interface.

Let K={(x,y):x≥0, y≥0, x+y≤1}, let f=1{x+y≤1/2}, and mark this single cell with θO=1. Use the uniform three-generation refinement, any estimator marking fraction, and a per-call budget sufficient for completion.

Facts & Assumptions

[A1]

Countable Choice is the background assumption in the cited Sobolev and L2 suppliers (The Axiom of Countable Choice (ACω)).

Given: Countable Choice, The P0 cell oscillation and the three-generation refinement rule.

[F1]

Cell oscillation is hK2∥f−ΠK0f∥22 (Residual indicators and data oscillation for P1 AFEM).

[F2]

The marked triangle's edge midpoints and the midpoint on its median occur in the three-generation NVB refinement (Shape regularity and local reduction under 2D newest-vertex bisection).

[F3]

The Dörfler-marked three-generation refinement reduces oscillation by at least the factor γO=1−34θO (Data oscillation reduction under three-generation refinement).

Verification

1.1A1F1givenalgebra

The triangle has area 1/2, while the subtriangle where f=1 has area 1/8, so ΠK0f=1/4. Therefore ∥f−ΠK0f∥22=(1/2)(1/4)(3/4)=3/32. Since diam⁡K=2, the initial oscillation square is 3/16.

2.1F2F3step 1.1algebra∎

The region x+y≤1/2 is the corner triangle whose other two vertices are the midpoints of the coordinate-axis edges. By [F2] its boundary is a union of refined edges; each refined leaf lies wholly in this region or its complement. Thus f is constant on every child, every child P0 projection is exact, and the refined oscillation is zero. This verifies the strict reduction and agrees with [F3], which gives γO=1/4 at θO=1.

Finite check

The parent mean is 1/4 and its variance integral is 3/32; the child partition aligns with the indicator boundary, so each child variance is zero.

Sources