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Three-dimensional Laplace cluster expansion error and work
Statement
For the single-cluster evaluator of Single-cluster three-dimensional Laplace multipole evaluator, let . If a target satisfies , then its potential error obeys If a target region is contained in a ball of radius centered at , with , then every target in the region has error at most Let and let be the number of targets. Under the monomial-coefficient evaluation convention of the algorithm, forming moments and evaluating the truncated field costs arithmetic operations after the basis coefficient arrays have been prepared. If all basis values at sources and targets are supplied, the coefficient accumulation and expansion evaluation alone cost . This is a single-cluster estimate; it is not a 3D multilevel FMM tree complexity theorem.
Facts & Assumptions
Given: A finite 3D source cluster in a ball , an orthonormal basis of each through degree , and targets strictly outside .
Degree-indexed spherical harmonic spaces in three dimensions: is the degree- homogeneous harmonic polynomial space restricted to the unit sphere, with the surface-area inner product; its regular solid harmonics are homogeneous degree- polynomials.
Three-dimensional Laplace spherical-harmonic addition theorem: the normalized spherical-harmonic addition identity and hold, and for .
Single-cluster three-dimensional Laplace multipole evaluator: moments are , and the output sums the corresponding basis expansion through degree .
Proof
For a source , put , , and when . For a target put and . Applying the generating identity and then the addition identity in [F2] with ratio gives . For , the same formula follows by the limit: the degree-zero term is and all positive-degree regular solid harmonics vanish at the center.
The dimension result in [F2] gives coefficients. At degree , each homogeneous polynomial has at most monomials and there are basis polynomials. Evaluating all basis values at one point by the prescribed monomial sums therefore uses at most a constant times operations.
Summing the finite source set with weights and grouping the finite sums over produces exactly the moments and the truncated evaluator in [F3]. The remainder magnitude is at most , using [F2] and the geometric-series formula.
If , the triangle inequality gives . The function , equal to , is nonincreasing for because its denominator is nondecreasing; substituting in step 2.1 gives the stated uniform target-region bound.
Repeating this basis evaluation at source points and target points costs . The remaining moment and target accumulations use coefficients per point, hence additional operations; if basis values are supplied, this latter count is the full coefficient-stage cost.
Depends on
Used by
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Sources
- Michael O’Neil, Integral Equations and Fast Algorithms, complete course lecture notes (2017) (standard reference, not scraped)