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Bounded mesh ratio gives an h^{-2s} exact condition-number bound
Statement
Assume Countable Choice. Fix Matérn parameters , , , , an admitted bounded domain , and a mesh-ratio bound . There is such that every finite nonempty distinct site set with , , , and satisfies The one-site case has condition number . This is an exact-arithmetic condition-number estimate for the Gram matrix; it does not bound the error of a floating-point solver.
Facts & Assumptions
Given: Countable Choice, fixed parameters, a bounded domain, a finite distinct site set with , and the stated mesh-ratio bound.
For and separation radius , ; for the condition number is . (Separated Matérn sites have a positive Gram lower bound)
The domain lies in a fixed ball and the disjoint balls of radius about the sites have volume proportional to ; finite additivity gives the corresponding packing bound. (Radial kernels, polynomial augmentation, and site geometry, Euclidean balls have positive finite Lebesgue measure, Lebesgue outer measure, Lebesgue measurability and Lebesgue measure are unchanged by translation, Assuming countable choice, is a sigma-algebra containing every elementary set and is a complete measure extending elementary volume, A linear map of sends Lebesgue measurable sets to Lebesgue measurable sets, with when is invertible and Lebesgue null when it is not)
The mesh ratio and separation radius satisfy , and the condition number is finite for each invertible Gram matrix. (Radial kernels, polynomial augmentation, and site geometry, The condition number kappa_p(A) = ||A||_p ||A^{-1}||_p of a nonsingular linear system)
Countable Choice is inherited from the Matérn separation estimate [F1] and the Lebesgue measure and ball-packing suppliers used in [F2]. (The Axiom of Countable Choice ())
Proof technique: packing followed by the separation estimate.
Proof
Given: The hypotheses above.
Suppose and write . Choose and with . The site balls are pairwise disjoint and lie in . Their measures are by [F2], so finite additivity and the positive finite unit-ball measure give and therefore .
Since , combine [F1] with step 1.1 to get .
The mesh-ratio bound implies , so . Step 2.1 gives the desired estimate.
Depends on
- Radial kernels, polynomial augmentation, and site geometry
- Separated Matérn sites have a positive Gram lower bound
- Euclidean balls have positive finite Lebesgue measure
- Lebesgue outer measure, Lebesgue measurability and Lebesgue measure are unchanged by translation
- Assuming countable choice, $\mathcal{L}(\mathbb{R}^n)$ is a sigma-algebra containing every elementary set and $\lambda_n$ is a complete measure extending elementary volume
- A linear map $T$ of $\mathbb{R}^n$ sends Lebesgue measurable sets to Lebesgue measurable sets, with $\lambda_n(T[E])=|\det T|\,\lambda_n(E)$ when $T$ is invertible and $T[E]$ Lebesgue null when it is not
- The condition number kappa_p(A) = ||A||_p ||A^{-1}||_p of a nonsingular linear system
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
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Sources
- Narcowich, Ward, and Wendland, Sobolev bounds on functions with scattered zeros, Math. Comp. 74 (2005), §3.1 (standard reference, not scraped)
- Gregory E. Fasshauer, MATH 590: Meshfree Methods, Chapter 13, pp. 34–36 (standard reference, not scraped)