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Weak nonparametric Bernstein–von Mises limits — Examples

1 · Prerequisites

2 · Summary

The examples compute the trace threshold for the negative Sobolev noise law and show why white noise cannot be treated as an L2 random function. A second example checks every hypothesis for a Gaussian wavelet product prior and an explicit nonzero truth, yielding the stated weak BvM limit.

3 · Logical flowchart

4 · Definitions, theorems and proofs

None yet.

5 · Examples, counterexamples and false statements

ExampleConstruction: AI-generatedVerification: AI-generatedprecheck passOpen item page →

White noise has a negative-Sobolev law but no L2-valued law

Example

Take a wavelet basis with 2l coefficients at level l and al=2l for l≥1. The weighted white-noise trace at δ=1/2 has level-l contribution

2l 2−l(log⁡2l)−1=1llog⁡2,

so the covariance trace used by this construction diverges. Under countable choice, for every δ>1/2 the trace is finite and white noise has a Borel Gaussian law on H(δ); under full AC its law is tight. In every infinite-dimensional orthonormal basis, white noise still fails to be an L2-valued random element. This example records the topology and target-law boundary in the weak BvM theorem; it does not assert a separate impossibility theorem for every conceivable L2 posterior limit.

Facts & Assumptions

Given: The wavelet-level weights and the standard independent white-noise coordinates.

[F1]

Logarithmic negative Sobolev spaces for white noise: For wavelets the level multiplicity is 2l and the weight is 2−l(log⁡al)−2δ, with the coarsest finite level treated separately.

[F2]

White noise is a Borel Gaussian law on the negative Sobolev space: Under countable choice, the white-noise series is Borel Gaussian in H(δ) for δ>1/2, while its independent standard-normal coefficients cannot be those of an L2 element.

[F3]

The canonical white-noise law is tight under the Axiom of Choice: Under AC, the canonical Borel law on Polish H(δ) is tight.

[A1]

The Axiom of Choice: Full AC is assumed only for the tightness conclusion in [F3]; the trace calculation and independent-coordinate obstruction to an L2 realization use no choice axiom.

[A2]

The Axiom of Countable Choice (ACω): Countable choice is assumed for the density-normal calculus interface in [F2]; its Borel Gaussian conclusion is used under this assumption.

Verification

Given: Countable choice, the basis, the weights, and the white-noise coefficient sequence; full AC is additionally assumed for tightness.

1.1F1algebra

At δ=1/2, the lth trace contribution for l≥1 is 1/(llog⁡2). On each block 2m≤l<2m+1 its sum is at least 1/(2log⁡2), so the trace diverges. For δ>1/2 the contribution is proportional to l−2δ; on each dyadic block its sum is at most a constant times 2m(1−2δ), a summable geometric sequence. Finite coarsest terms do not affect either conclusion.

1.2A1A2F2F3

For δ>1/2, [F2] under [A2] gives the Borel Gaussian law on H(δ) and the failure of an L2 realization; [F3] gives tightness of that law under [A1].

2.1A1A2F2F3step 1.1step 1.2∎

Therefore, under countable choice, the canonical Gaussian target for the weak BvM result is the negative-Sobolev law described in [F2], tight under the additional full-AC branch [F3]; this target-law construction makes no separate claim about posterior convergence in L2 or total variation.

ExampleConstruction: AI-generatedVerification: AI-generatedprecheck passOpen item page →

A Gaussian wavelet product prior covered by the weak BvM theorem

Example

Let φ(u)=(2π)−1/2e−u2/2 be the standard normal density, take any α>0, and use the product prior scales σl=2−l(α+1/2) on an S-regular wavelet basis with 2l coefficients at level l≥0, as in the weak BvM theorem. Let the truth have coefficients θ0,lk=σl, so rlk=1 for every (l,k). With M=1 and τ=2, all density and coefficient hypotheses of the weak BvM theorem hold.

Facts & Assumptions

Given: The stated S-regular wavelet basis, the standard normal coefficient density, the scale σl, and the truth coefficients θ0,lk=σl.

[F1]

The wavelet product prior is L2-valued: For α>0 and a coefficient density with finite second moment, the wavelet series defines an L2 random element.

[F2]

Logarithmic negative Sobolev spaces for white noise: The chosen S-regular wavelet system is a complete orthonormal basis of L2([0,1]).

[F3]

Riesz-Fischer completeness of Lp for 1≤p≤∞: Under countable choice, L2([0,1]) is complete.

[F4]

Density normal laws and square-summable Gaussian series under countable choice: The standard normal density has mean 0 and variance 1.

[F5]

Weak nonparametric Bernstein–von Mises theorem for product priors: For the stated S-regular wavelet basis, under countable choice and with the supplied white-noise model and posterior kernel, the weak BvM conclusion holds when the truth's normalized coefficients are bounded by M<τ, the prior density is bounded above and bounded below on (−τ,τ), has finite second moment, and is continuous at each normalized truth coefficient.

[A1]

The Axiom of Countable Choice (ACω): Countable choice supplies the L2 completeness in [F3] and the prior construction in [F1]; the full posterior kernel is supplied by the model input of [F5], and no full Axiom of Choice is needed for the bounded-Lipschitz conclusion.

Verification

Given: Countable choice, the stated S-regular wavelet basis, α>0, σl=2−l(α+1/2), θ0,lk=σl, φ(u)=(2π)−1/2e−u2/2, and the white-noise experiment and supplied full posterior kernel of [F5].

1.1F4givenalgebra

The standard normal density is bounded and continuous, and [F4] gives its second moment 1. For ∣u∣<2, e−u2/2>e−2, so φ(u)≥(2π)−1/2e−2>0. The normalized truth coefficients all equal rlk=1, hence sup⁡l,k∣rlk∣=1≤M=1<τ=2.

1.2A1F1F2F3givenalgebra

Let fN=∑0≤l≤N∑k∈Klσlψlk. By [F2], for M>N orthogonality gives ∥fM−fN∥22=∑N<l≤M2lσl2=∑N<l≤M2−2αl, whose tail tends to zero. Completeness [F3] and [A1] give a limit f0∈L2([0,1]); continuity of each coefficient functional yields ⟨f0,ψlk⟩=σl. The prior is likewise L2-valued by [F1] under [A1].

2.1A1F2F5givenstep 1.1step 1.2∎

With the stated S-regular basis and supplied posterior kernel, all hypotheses of [F5] now hold, including continuity at each rlk=1; therefore for every δ>1/2 the scaled posterior converges to canonical white noise in the bounded-Lipschitz metric of H(δ), in probability under the fixed-truth data law.

Sources