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A smooth scattered surface inherits the Matérn fill-distance rate
Example
Assume the Axiom of Choice and Countable Choice as required by the Fourier-space and interpolation suppliers. Fix , , , and let . The smooth graph with is interpolated from midpoint-grid samples by the normalized Matérn kernel of smoothness .
Facts & Assumptions
Given: Full AC, Countable Choice, the fixed , the cube, the function , and midpoint grids
An admitted domain is nonempty, bounded and Lipschitz and has a single cone radius and aperture at every point of the closure; the fill distance is . (Radial kernels, polynomial augmentation, and site geometry)
Every Schwartz function is in the Fourier Bessel-potential space for every , and consists of restrictions of whole-space members. (Schwartz functions are dense in the Fourier Bessel-potential norm, Fourier Bessel-potential and restriction-quotient Sobolev spaces)
The real exponential is smooth with derivative itself; repeated coordinatewise chain and product rules show every partial derivative of is a finite sum of polynomial multiples of . (The exponential function is smooth and , The chain rule, in one line from Carathéodory: if is differentiable at and is differentiable at , then is differentiable at with , Sums, scalar multiples, products and quotients: , , , and when )
The Schwartz seminorms require polynomially weighted derivatives to be bounded. For any degree , take an integer with ; for , and exponential domination applied with gives . On , each polynomial is bounded by the sum of the absolute values of its finitely many coefficients. (Schwartz space and its seminorms, The exponential dominates every fixed nonnegative integer power at )
The Matérn interpolation error and power function are bounded by on an admitted cone domain for small fill distance. (Matérn interpolation and its power function converge at rate h^ν)
Proof technique: verify the domain, regularity, and grid radius, then apply the fill-distance theorem.
Verification
Given: The data above.
The cube is bounded and Lipschitz. At a boundary point , let be the nonempty set of active coordinates ( or ), with inward signs at and at . Put for and otherwise. In the orthogonal coordinates with , a sufficiently small neighborhood has no constraints from inactive coordinates, and the active inequalities reduce to This is the epigraph of a finite maximum of linear functions, hence a local Lipschitz graph with the cube on one side. The cube also satisfies the uniform cone condition: for , let if and otherwise, and put . If let ; otherwise put and , which is orthogonal and sends to . Choose with , and set . For any cone vector , and . Hence for every coordinate, . Each coordinate moves toward the cube's interior by less than , so .
The sites in are distinct. Each is at coordinatewise distance at most from a midpoint in each grid interval, so . By [F3], every partial derivative of is a finite sum of polynomial multiples of ; [F4] makes every polynomially weighted derivative bounded, so is Schwartz. Then [F2] gives for .
For all sufficiently large , step 2.1 gives from [F5]. The Matérn interpolation theorem therefore yields The graph samples are the scattered surface data, and the bound uses fill distance alone; no mesh-ratio condition is needed.
Depends on
- Radial kernels, polynomial augmentation, and site geometry
- The normalized Matérn kernel in the 2π Fourier convention
- Fourier Bessel-potential and restriction-quotient Sobolev spaces
- The Matérn spectral integral, normalization, and radial formula
- Schwartz functions are dense in the Fourier Bessel-potential norm
- Schwartz space and its seminorms
- The exponential function is smooth and $(\exp)'=\exp$
- The chain rule, in one line from Carathéodory: if $g$ is differentiable at $c$ and $f$ is differentiable at $g(c)$, then $f \circ g$ is differentiable at $c$ with $(f \circ g)'(c) = f'(g(c))\,g'(c)$
- Sums, scalar multiples, products and quotients: $(f+g)'(c) = f'(c) + g'(c)$, $(\alpha f)'(c) = \alpha f'(c)$, $(fg)'(c) = f'(c)g(c) + f(c)g'(c)$, and $(f/g)'(c) = \bigl(f'(c)g(c) - f(c)g'(c)\bigr)/g(c)^{2}$ when $g(c) \ne 0$
- The exponential dominates every fixed nonnegative integer power at $+\infty$
- Matérn interpolation and its power function converge at rate h^ν
- The Axiom of Choice
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
Used by
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Sources
- Narcowich, Ward, and Wendland, Sobolev bounds on functions with scattered zeros, Math. Comp. 74 (2005), Theorem 1.1 (standard reference, not scraped)
- Gregory E. Fasshauer, MATH 590: Meshfree Methods, Chapter 13, pp. 34–36 (standard reference, not scraped)