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Gaussian shift LAN and the regular sample mean

Statement

Let X1,…,Xn be IID N(θ,1) and take θ0=0. This one-dimensional model is QMD at zero with score s(x)=x and information I=1. For every fixed h, the local laws Pn,h=N(h/n,1)⊗n are exactly dominated by Pn,0 and log⁡dPn,hdPn,0(X1,…,Xn)=hΔn−12h2,Δn=1n∑i=1nXi. The sample mean estimator is regular, with n(Xˉn−hn)⇒N(0,1) under Pn,h for each fixed h.

Facts & Assumptions

Given: AC; the standard normal density ϕ(x)=(2π)−1/2e−x2/2; independent observations; and the definitions of QMD, LAN, and regularity.

[A1]

The Axiom of Choice: AC is assumed by the normal-law and characteristic-function uniqueness suppliers.

[F1]

Standard normal and normal laws and The standard normal density has total mass one: The density ϕ defines the probability law N(0,1).

[F2]

Characteristic function of a normal law: If X∼N(m,σ2), then EX=m, Var⁡(X)=σ2, and its characteristic function is exp⁡(imt−σ2t2/2).

[F3]

Characteristic functions under affine maps and independent sums: The characteristic function of an independent sum is the product of the component characteristic functions, with the corresponding affine scaling.

[F4]

Uniqueness of a law from its characteristic function: Probability laws with the same characteristic function are equal.

[F5]

Dominated convergence: A pointwise limit dominated in absolute value by one integrable function passes through the Lebesgue integral.

[F9]

Quadratic-mean differentiability at a parameter, Local asymptotic normality for local experiments, and Regular estimator under local alternatives: QMD is the square-root density L2 expansion, LAN is the local likelihood expansion with Gaussian central sequence, and regularity requires one common centered limit law for each fixed local h.

[F10]

Quadratic-mean differentiability implies iid LAN: A QMD IID model has the LAN expansion with its score and information, using mean-one proxies when needed.

[F11]

Likelihood densities: The Gaussian densities used here are measurable densities with respect to Lebesgue measure and give the stated probability laws by integration.

Proof

Proof technique: verify square-root differentiability and compute the local likelihood ratio exactly.

1.1F1F2F5F6F7F11givenalgebra

Let gθ(x)=ϕ(x−θ)=(2π)−1/4e−(x−θ)2/4. By [F6] its derivative is ∂θgθ(x)=12(x−θ)gθ(x), so ∂θg0(x)=12xg0(x). For ∣θ∣≤1, ∣∂θgθ(x)∣≤C(1+∣x∣)e−((∣x∣−1)+)2/4=:G(x), whose square is integrable: it is bounded on [−2,2], and for ∣x∣≥2 it is bounded by C′(1+x2)e−x2/8. Substituting x=2y reduces the tail integral to a constant times ∫(1+4y2)e−y2/2dy, finite by the second moment statement in [F2]. By [F7], (gt−g0)/t=∫01∂θgrt dr; this converges pointwise to 12xg0. Its difference is bounded by 2G, so [F5] applied to the squared difference gives convergence in L2(dx). Thus the QMD residual has norm o(∣t∣), which is exactly the scalar QMD condition.

1.2F1F2F3F4F9F11givenalgebra

The density ratio is ∏i=1nϕ(Xi−h/n)ϕ(Xi)=exp⁡ ⁣(hn∑i=1nXi−h22). Each density is positive, so this is a full mean-one likelihood ratio. By [F2]–[F4], the characteristic function of Δn is ∏i=1nexp⁡(−t22n)=e−t2/2, so Δn∼N(0,1) exactly. Comparing with [F9] proves exact LAN with K=1 and no remainder.

2.1F2F10step 1.1algebra

The score is s(x)=x. By [F2], E0s=0 and E0s2=1, so the information is I=1. By [F10], the general QMD-to-LAN result applies and predicts the same score and information; the direct computation below also shows the proxy is the exact alternative law.

3.1A1F2F3F4F9givenalgebra∎

Under Pn,h, the variables Xi−h/n are IID N(0,1). The same characteristic-function product using [F3]–[F4] gives n(Xˉn−hn)=1n∑i(Xi−h/n)∼N(0,1) exactly, which implies the displayed convergence. The limiting law does not depend on fixed h, which is regularity by [F9]. The calculation includes h=0 and every finite h.

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