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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 Q=∑j∈J∣qj∣. If a target satisfies r=∣x−c∣>a, then its potential error obeys ∣U(x)−U~p(x)∣≤Qr(a/r)p+11−a/r. If a target region is contained in a ball of radius b centered at d, with R=∣d−c∣>a+b, then every target in the region has error at most QR−b(a/(R−b))p+11−a/(R−b). Let S=∣J∣ and let T0 be the number of targets. Under the monomial-coefficient evaluation convention of the algorithm, forming moments and evaluating the truncated field costs O((S+T0)(p+1)4) 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 O((S+T0)(p+1)2). 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 B(c,a), an orthonormal basis of each Hℓ through degree p, and targets strictly outside B(c,a).

[F1]

Degree-indexed spherical harmonic spaces in three dimensions: Hℓ is the degree-ℓ homogeneous harmonic polynomial space restricted to the unit sphere, with the surface-area inner product; its regular solid harmonics are homogeneous degree-ℓ polynomials.

[F2]

Three-dimensional Laplace spherical-harmonic addition theorem: the normalized spherical-harmonic addition identity and ∣Pℓ(t)∣≤1 hold, and ∑ℓ≥0rℓPℓ(t)=(1−2rt+r2)−1/2 for 0≤r<1.

[F3]

Single-cluster three-dimensional Laplace multipole evaluator: moments are Mℓm=∑jqjHℓm(yj−c)‾, and the output sums the corresponding basis expansion through degree p.

Proof

1.1givenF1F2algebra

For a source yj, put vj=yj−c, rj=∣vj∣≤a, and ωj=vj/rj when rj>0. For a target put r=∣x−c∣>a and ω=(x−c)/r. Applying the generating identity and then the addition identity in [F2] with ratio rj/r<1 gives 1∣x−yj∣=∑ℓ=0∞rjℓrℓ+1Pℓ(ω⋅ωj)=4π∑ℓ=0∞12ℓ+1rjℓrℓ+1∑m=−ℓℓYℓm(ω)Yℓm(ωj)‾. For rj=0, the same formula follows by the limit: the degree-zero term is 1/r and all positive-degree regular solid harmonics vanish at the center.

1.2givenF1F2algebra

The dimension result in [F2] gives ∑ℓ=0p(2ℓ+1)=(p+1)2 coefficients. At degree ℓ, each homogeneous polynomial has at most (ℓ+22) monomials and there are 2ℓ+1 basis polynomials. Evaluating all basis values at one point by the prescribed monomial sums therefore uses at most a constant times ∑ℓ=0p(2ℓ+1)(ℓ+22)=O((p+1)4) operations.

2.1givenF2F3step 1.1algebra

Summing the finite source set with weights qj and grouping the finite sums over m produces exactly the moments and the truncated evaluator in [F3]. The remainder magnitude is at most ∑j∣qj∣∑ℓ=p+1∞rjℓrℓ+1∣Pℓ(ω⋅ωj)∣≤Qr∑ℓ=p+1∞(a/r)ℓ=Qr(a/r)p+11−a/r, using [F2] and the geometric-series formula.

3.1givenstep 2.1algebra

If ∣x−d∣≤b, the triangle inequality gives r=∣x−c∣≥R−b>a. The function ap+1/(rp+1(r−a)), equal to r−1(a/r)p+1/(1−a/r), is nonincreasing for r>a because its denominator is nondecreasing; substituting r=R−b in step 2.1 gives the stated uniform target-region bound.

4.1F3step 1.2algebra∎

Repeating this basis evaluation at S source points and T0 target points costs O((S+T0)(p+1)4). The remaining moment and target accumulations use (p+1)2 coefficients per point, hence O((S+T0)(p+1)2) additional operations; if basis values are supplied, this latter count is the full coefficient-stage cost.

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