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Smooth periodic samples have a best m-term D4 Besov-tail rate

Statement

Let J≥3, N=2J, and let f:R→R be 1-periodic and C2, with f∣U∈S2 for an open interval U⊃[0,1]. Form the normalized sample vector xn=N−1/2f(n/N) and its multilevel D4 coefficients (a,d). For any 0<s<2, set p=(s+1/2)−1. Then ∥(a,d)∥bp,ps(J)p≤4M0p+(93M2/2)p 2−2p(2−s)1−2−p(2−s), where Mr=sup⁡[0,1]∣f(r)∣, and for 1≤m≤N, ∥x−UNTHm(UNx)∥2≤p2−p(4M0p+(93M2/2)p 2−2p(2−s)1−2−p(2−s))1/pm−s. The norm is the finite wavelet-coefficient convention of Finite wavelet-coefficient Besov quasi-norm on a dyadic multilevel transform; no intrinsic Besov norm equivalence is asserted.

Facts & Assumptions

Given: A 1-periodic C2 function f:R→R, its restriction f∣U∈S2 on an open interval U⊃[0,1], its normalized samples, and s∈(0,2).

[F1]

Two D4 vanishing moments give a local coefficient bound for smooth periodic samples: each detail coefficient at scale j is bounded by (93/2)M2 2−5j/2.

[F2]

Periodic multilevel transform using the four-tap Daubechies filter: the coarse block has four coefficients, and the level-j detail block has 2j coefficients.

[F3]

Inverse and Parseval identity for a periodic orthogonal compact-filter transform: the multilevel analysis matrix UN is orthogonal and preserves the Euclidean norm.

[F4]

Finite wavelet-coefficient Besov quasi-norm on a dyadic multilevel transform: the critical coefficient norm has no scale weights when p=(s+1/2)−1.

[F5]

Critical Besov coefficient control implies a best m-term squared-error rate: a finite orthogonal coefficient vector with this critical norm has best m-term error at most p/(2−p)∥c∥bp,psm−s.

Proof

1.1givenF2F3F4algebra

Here 1/p=s+1/2, so the coefficient norm's scale weight is 1 by [F4]. Orthogonality [F3] gives ∥x∥22=N−1∑n=0N−1∣f(n/N)∣2≤M02, and therefore each of the four coarse coefficients has magnitude at most M0.

1.2givenF1F2F4algebra

At scale j, [F1] and [F2] imply ∑k=02j−1∣dj,k∣p≤2j(93M2/2)p2−5jp/2=(93M2/2)p2−jp(2−s), because 1−p(5/2)=−p(2−s). Summing over the finite range 2≤j≤J−1 and bounding by the infinite geometric series gives the first displayed estimate. Its ratio is 2−p(2−s)<1 since 0<s<2.

2.1givenF3F5step 1.2algebra∎

Apply [F5] to the finite orthogonal coefficient vector of UNx and substitute the first bound from step 1.2. This gives the stated best m-term error rate with the declared constant.

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