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Tapered block bootstrap CLT for dependent Hilbert-valued data

Statement

Assume the Axiom of Choice. Let H=L2([0,1]), and let (Yt)t∈Z be a centered causal Bernoulli shift that is L2-m-approximable under L2-m-approximable causal Bernoulli shifts. Write

δm=(E∥Y0−Y0,m∥H2)1/2.

By Independent-tail replacement errors are decreasing and summability implies the rate condition, the summability in the stated process definition implies mδm→0. Let the integers bn→∞ with bn/n→0. Let the taper satisfy the support, positivity-near-1/2, symmetry and monotonicity conditions in Tapered block bootstrap. For all sufficiently large n, 1≤bn≤n and Wbn>0, the latter by Asymptotic squared-norm normalization of a tapered window. Apply that algorithm to Y1,…,Yn only on this admissible tail; all limits below concern that tail. Write Yˉn∗=n−1∑t=1nYt∗ for the resulting bootstrap mean. Let Γ be the long-run covariance operator from Hilbert-valued CLT for L2-m-approximable Bernoulli shifts. Then

L∗ ⁣(n(Yˉn∗−E∗Yˉn∗))⇒L(G)in probability,

and

∥n Cov⁡∗(Yˉn∗)−Γ∥HS⟶0in probability,

where G is centered Gaussian with covariance Γ.

Facts & Assumptions

Given: AC; the centered causal shift with summable replacement errors; the stated block regime; and the admissible taper.

[F1]

L2-m-approximable causal Bernoulli shifts supplies the measurable causal representation, second moment and summable replacement errors.

[F2]

Hilbert-valued CLT for L2-m-approximable Bernoulli shifts supplies the sampling-law limit and centered Gaussian G of covariance Γ; it makes no time-indexed FCLT assertion.

[F3]

Tapered block bootstrap uses midpoint weights cb,ℓ=abw((ℓ−1/2)/b) on sample-centered observations, independent uniform starts and truncation to exactly n observations. Exact conditional covariance of a tapered block bootstrap mean gives the exact conditional full-plus-partial covariance.

[F4]

Asymptotic squared-norm normalization of a tapered window gives Wb2/b→Iw>0, so ab=b/Wb is defined and bounded for all sufficiently large b.

[F5]

Weighted overlapping-block moment and covariance limits proves uniform integrability and empirical trace-norm covariance consistency for bounded normalized weights with fixed-lag autocorrelations tending to one. Conditional CLT for independent resampled weighted blocks proves the corresponding conditional law, tightness, trace-norm covariance limit and partial-block bound; subtracting any common sample center leaves its centered blocks unchanged.

[F6]

Under the independent-tail construction in [F1], summability of the replacement errors implies mδm→0 (Independent-tail replacement errors are decreasing and summability implies the rate condition).

[A1]

The Axiom of Choice is assumed. Its uses are inherited from the sampling and conditional-block suppliers, including the latter's complete orthonormal basis; no further choice principle is added.

Proof

Given: The assumptions and suppliers above.

1.1givenF1F3F4F5F6A1algebra

Set cb,ℓ=abw((ℓ−1/2)/b) and extend this finite sequence by zero on all other integer indices. The exact normalization gives b−1∑ℓcb,ℓ2=1, and [F4] bounds its maximum by a constant A for sufficiently large b. Symmetry and monotonicity of w make the sequence nonnegative and unimodal, including the possible equal central pair; hence its discrete total variation, including both exterior zero tails, is at most 2A. For every fixed integer h≥1, telescoping h successive shifts gives ∑ℓ∈Z∣cb,ℓ+h−cb,ℓ∣≤2Ah. Therefore ∣b−1∑ℓ=1b−hcb,ℓcb,ℓ+h−1∣≤Ab∑ℓ∈Z∣cb,ℓ+h−cb,ℓ∣≤2A2hb⟶0. This verifies every weight hypothesis of [F5], even for discontinuous monotone tapers. By [F6], the process satisfies the replacement-error rate needed by [F5].

2.1F3F5step 1.1algebra

For each full or partial weighted block, replacing Yt by Yt−Yˉn subtracts (∑ℓ=1qcb,ℓ)Yˉn from every candidate block with q entries. Its empirical conditional center over the N=n−b+1 starts subtracts the same vector, so all conditionally centered block sums agree exactly with those formed from Yt. Thus n(Yˉn∗−E∗Yˉn∗) is precisely Zn∗ in [F5], with the full-taper prefix weights on its last r=n−b⌊n/b⌋ entries.

3.1F2F3F5step 1.1step 2.1∎

Apply [F5] with the sampling-law Gaussian G supplied by [F2]. It gives conditional weak convergence to that same G, conditional tightness, and convergence of the exact covariance nCov⁡∗(Yˉn∗) in trace norm, which implies its stated Hilbert-Schmidt convergence. The final prefix has expected conditional squared norm O(b/n) and is negligible without requiring b∣n. Its limiting covariance Γ is the sampling target in [F2], completing both claims.

The paper's Assumption 1 states mδm→0; the local independent-tail replacement lemma derives this rate from summability under the stated replacement convention, so the local block argument adds no process restriction.

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