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Finite-prior Bayes envelopes converge under pointwise LAN
Statement
Assume AC. In a pointwise LAN experiment Local asymptotic normality for local experiments with , fix and weights summing to one. Let be Lipschitz. The Bayes values for estimating with loss and this finite prior, allowing all measurable randomized rules, converge to the corresponding Bayes value in the Gaussian shift . No likelihood-field uniformity or measurable-selection hypothesis is required.
Facts & Assumptions
Given: AC, the pointwise LAN experiment, finite prior and bounded Lipschitz loss.
Local asymptotic normality for local experiments gives mean-one likelihood proxies , pointwise total-variation approximation, and with . Its finite log expansion also gives .
Vector Slutsky theorem and Continuous mapping theorem give joint finite-dimensional convergence after adjoining finitely many remainders that tend to zero in probability.
A mean-one likelihood-ratio limit implies contiguity proves uniform integrability for nonnegative mean-one variables converging in law to a mean-one limit.
Bayes risk and minimax risk defines the Bayes risk of a rule as , while Measure kernel and probability kernel describes randomized rules. In this proof the Bayes value means the infimum of over all measurable randomized rules; the infimum is local notation and is not part of the supplier's definition.
Total variation distance for probability laws and Tonelli's theorem for nonnegative measurable functions on a sigma-finite product imply for , by integrating the tail probabilities of .
Both and are dense in , and every nonempty open subset of is uncountable gives a countable dense rational grid in finite-dimensional Euclidean action space by taking finite products.
Multivariate normal law, including singular covariance, Standard normal and normal laws, The standard normal density has total mass one, and A linear map of sends Lebesgue measurable sets to Lebesgue measurable sets, with when is invertible and Lebesgue null when it is not: For with , its density is proportional to , and completing the square shows .
The Lebesgue integral is linear on : Expectation commutes with the finite weighted sums defining and .
The Axiom of Choice is retained for the LAN and Gaussian suppliers.
Proof
For , set . For any , each action's objective changes by at most ; taking infima proves the same bound for . Also . For fixed the objective is continuous in , so F6 makes its infimum equal to the infimum over a fixed enumeration of the rational grid. For , choose the first whose objective is at most . This index is Borel: its defining set and the finitely many failures at smaller indices are Borel. Thus this is an explicit measurable -optimal action function of .
Under the base law, F1 and F2 imply , where and . The log expansions imply that zero likelihoods have vanishing probability, so exponentiating the finite vector is legitimate. By [F7], completing the square in the Gaussian density gives . Hence converges in law to , and [F8] gives . [F3] makes uniformly integrable.
For each cutoff , is bounded continuous, so its expectations converge by step 1.2. The error from this cutoff is bounded by , which vanishes uniformly as ; the limit variable has the same integrable bound. Therefore .
Under the proxy mixture, the predictive density is and, when , the posterior weights are . Conditional on the data, every randomized action rule has posterior risk at least , because is the infimum over deterministic actions and randomization averages those action risks. The measurable selector from step 1.1 has posterior risk at most this envelope plus . Multiplying by the predictive density and integrating shows that the proxy Bayes value differs from by at most ; the set has zero predictive probability. Letting identifies the proxy Bayes value with that expectation. The same argument with identifies the limiting envelope expectation as the finite-prior Gaussian Bayes value: tilting the base Gaussian by gives , and has law . Finally [F5] bounds the actual/proxy risk difference uniformly over rules by . Infima preserve this bound, proving the assertion.
Depends on
- Local asymptotic normality for local experiments
- Vector Slutsky theorem
- Continuous mapping theorem
- A mean-one likelihood-ratio limit implies contiguity
- Bayes risk and minimax risk
- Measure kernel and probability kernel
- Total variation distance for probability laws
- Tonelli's theorem for nonnegative measurable functions on a sigma-finite product
- Both $\mathbb{Q}$ and $\mathbb{R} \setminus \mathbb{Q}$ are dense in $\mathbb{R}$, and every nonempty open subset of $\mathbb{R}$ is uncountable
- The Axiom of Choice
- Multivariate normal law, including singular covariance
- Standard normal and normal laws
- The standard normal density has total mass one
- A linear map $T$ of $\mathbb{R}^n$ sends Lebesgue measurable sets to Lebesgue measurable sets, with $\lambda_n(T[E])=|\det T|\,\lambda_n(E)$ when $T$ is invertible and $T[E]$ Lebesgue null when it is not
- The Lebesgue integral is linear on $L^1(\mu)$
Used by
- Local asymptotic minimax bound Theorem
Dependency tree · two levels
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Sources
- A. W. van der Vaart, Semiparametric Statistics, Theorem 4.14 and complete proof, pp. 46–48 (standard reference, not scraped)
- John Duchi, A few notes on contiguity, asymptotics, and local asymptotic normality, Theorem 3 and proof, pp. 17–18; local finite-prior proof supplied here (standard reference, not scraped)