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A Fourier integral missed by one sample but captured by Filon moments

Example

Let N be a positive odd integer, ω=2πN, and g(x)=x on [0,1]. For f(x)=1, the exact integral is 0 and the endpoint Filon rule returns 0, while the one-sample midpoint value Mω(f)=f(1/2)eiω/2 is −1. For f(x)=x2, the same endpoint Filon rule has the exact error 2/ω2.

Facts & Assumptions

Given: The positive odd integer N, ω=2πN, g(x)=x, and the two amplitudes f(x)=1 and f(x)=x2 on [0,1].

[F1]

Filon quadrature from nodal interpolation and phase moments: the Filon output is ∑jf(cj)μ^j, using the Lagrange interpolant's phase moments.

[F2]

Exact recurrence for polynomial Fourier moments: for endpoints a,b, the recurrence is μ0=(eiωb−eiωa)/(iω) and μk=(bkeiωb−akeiωa)/(iω)−kμk−1/(iω).

[F3]

Exact polynomial moments for the linear Fourier phase: each output μk is the exact integral ∫abxkeiωx dx.

[F4]

Nonstationary error bound for endpoint Filon interpolation: endpoint Filon with exact moments has error bounded by an explicit constant times ∣ω∣−2 under f∈C2 and nonstationary g∈C3.

[F7]

Laws of integer exponents: for a nonzero field element and natural exponents, (am)n=amn and am+n=aman.

Verification

1.1givenF1F2F3F5F6F7algebra

Since ei2πN=(eiπ)2N=(−1)2N=1 and eiπN=(−1)N=−1 for odd N, by [F5]--[F7], [F2] and [F3] give μ0=0. For f=1 the endpoint interpolant is constant, so [F1] returns the exact integral 0, while the one-sample value is Mω(1)=eiπN=−1.

2.1givenF1F2F3F4step 1.1algebra∎

For f=x2, the degree-one endpoint interpolant is p(x)=x. Step 1.1 gives eiω=1, so [F2] gives μ1=1/(iω) and μ2=1/(iω)−2/(iω)2. Thus Iω(f,g)−Q1=μ2−μ1=−2/(iω)2=2/ω2. The hypotheses of [F4] hold, so this exact error has the asserted second-order frequency decay.

Depends on

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