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Gaussian shift LAN and the regular sample mean
Statement
Let be IID and take . This one-dimensional model is QMD at zero with score and information . For every fixed , the local laws are exactly dominated by and The sample mean estimator is regular, with under for each fixed .
Facts & Assumptions
Given: AC; the standard normal density ; independent observations; and the definitions of QMD, LAN, and regularity.
The Axiom of Choice: AC is assumed by the normal-law and characteristic-function uniqueness suppliers.
Standard normal and normal laws and The standard normal density has total mass one: The density defines the probability law .
Characteristic function of a normal law: If , then , , and its characteristic function is .
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.
Uniqueness of a law from its characteristic function: Probability laws with the same characteristic function are equal.
Dominated convergence: A pointwise limit dominated in absolute value by one integrable function passes through the Lebesgue integral.
The exponential function is smooth and and The chain rule, in one line from Carathéodory: if is differentiable at and is differentiable at , then is differentiable at with : The real exponential differentiates by itself, and derivatives of compositions obey the chain rule.
The first fundamental theorem: if is integrable on and continuous at , then ; in particular a continuous has as a primitive: Integrating a continuous derivative along an interval recovers the endpoint difference.
Quadratic-mean differentiability at a parameter, Local asymptotic normality for local experiments, and Regular estimator under local alternatives: QMD is the square-root density expansion, LAN is the local likelihood expansion with Gaussian central sequence, and regularity requires one common centered limit law for each fixed local .
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.
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.
Let . By [F6] its derivative is , so . For , whose square is integrable: it is bounded on , and for it is bounded by . Substituting reduces the tail integral to a constant times , finite by the second moment statement in [F2]. By [F7], ; this converges pointwise to . Its difference is bounded by , so [F5] applied to the squared difference gives convergence in . Thus the QMD residual has norm , which is exactly the scalar QMD condition.
The density ratio is Each density is positive, so this is a full mean-one likelihood ratio. By [F2]–[F4], the characteristic function of is so exactly. Comparing with [F9] proves exact LAN with and no remainder.
The score is . By [F2], and , so the information is . 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.
Under , the variables are IID . The same characteristic-function product using [F3]–[F4] gives exactly, which implies the displayed convergence. The limiting law does not depend on fixed , which is regularity by [F9]. The calculation includes and every finite .
Depends on
- Quadratic-mean differentiability at a parameter
- Likelihood densities
- Quadratic-mean differentiability implies iid LAN
- Local asymptotic normality for local experiments
- Regular estimator under local alternatives
- Standard normal and normal laws
- The standard normal density has total mass one
- Characteristic function of a normal law
- Characteristic functions under affine maps and independent sums
- Uniqueness of a law from its characteristic function
- Dominated convergence
- The exponential function is smooth and $(\exp)'=\exp$
- The chain rule, in one line from Carathéodory: if $g$ is differentiable at $c$ and $f$ is differentiable at $g(c)$, then $f \circ g$ is differentiable at $c$ with $(f \circ g)'(c) = f'(g(c))\,g'(c)$
- The first fundamental theorem: if $f$ is integrable on $[a,b]$ and continuous at $c$, then $F'(c) = f(c)$; in particular a continuous $f$ has $F$ as a primitive
- The Axiom of Choice
Used by
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Sources
- Peter Bartlett, Theoretical Statistics, Stat 210B Lecture 23, PDF pp. 2–3 and 6–9 (standard reference, not scraped)
- John Duchi, A few notes on contiguity, asymptotics, and local asymptotic normality, §2.1 (standard reference, not scraped)