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Finite-dimensional LAN convolution bound for differentiable functionals

Statement

Assume AC and let d,k≥1 be finite. Let {Pn,h:h∈Rd} be LAN at h=0 with positive-definite information matrix K and central sequence Δn. Let ψn,h∈Rk be a local target such that for every fixed h,

δn(h):=n(ψn,h−ψn,0)⟶Dh

for a fixed matrix D∈Rk×d. Suppose Tn=n(ψ^n−ψn,0) is regular for this target: under each fixed Pn,h, Tn−δn(h)⇒W for the same Borel law W. Then there is a probability law ν on Rk such that

L(W)=Nk(0,DK−1DT)∗ν.

The Gaussian covariance may be singular. No finite-moment hypothesis on W or ν is needed.

Facts & Assumptions

Given: AC; the finite-dimensional LAN experiment, positive-definite K, central sequence Δn; the local target derivative matrix D; and a regular estimator with common limit law W.

[F1]

Local asymptotic normality for local experiments: for each fixed h, a mean-one proxy has log⁡Ln,h=hTΔn−12hTKh+oPn,0(1) and is total-variation close to the actual local law.

[F2]

Regular estimator under local alternatives: for an identity target the centered statistic has a common limit under each fixed local displacement; the statement gives the corresponding target-shift condition explicitly.

[F3]

Le Cam's third lemma for a Gaussian likelihood-ratio limit: the joint limit tilted by the mean-one Gaussian likelihood ratio describes the statistic's limit under the proxy local law.

[F4]

Weakly convergent sequences are tight and Prokhorov tightness theorem on polish spaces: convergent marginal laws are tight, and under AC a tight sequence of joint Borel laws on Euclidean space has a weakly convergent subsequence.

[F5]

Identity theorem for holomorphic functions: two holomorphic functions on a connected complex domain that agree on a set with an interior accumulation point agree throughout the domain.

[F6]

Multivariate normal law, including singular covariance and Characteristic function of a multivariate normal law: Nd(0,K) has characteristic function u↦e−uTKu/2 and finite exponential moments in every real direction.

[F7]

Continuous mapping theorem and Slutsky's theorem for real random variables: continuous projections preserve weak limits, and scalar oP(1) remainders preserve those limits.

[F8]

Uniqueness of a law from its characteristic function: real probability laws with equal characteristic functions are equal; the vector conclusion below is obtained from scalar projections using Cramer–Wold.

[F9]

Cramer wold device: under AC, equality or convergence of all real projections determines the Borel vector law.

[F12]

Finite and countable subadditivity of measures: the measure of a finite union is at most the sum of the measures of its members.

[F13]

Characteristic function of a real random variable: the characteristic function of a real random variable X is t↦E[eitX].

[F14]

Fubini's theorem for L^1 functions on a sigma-finite product: a bounded complex-valued function is integrable under a product probability, and Fubini factors the integral of a product of coordinate functions.

[A1]

The Axiom of Choice: AC is assumed by LAN, Prokhorov's theorem, and the finite-dimensional characteristic-function uniqueness and Cramer–Wold suppliers.

Proof

Given: The local experiment, target, estimator, and hypotheses above.

1.1A1F1F2F4F11F12givenchoose

Under Pn,0, Tn⇒W by regularity at h=0 and Δn⇒Nd(0,K) by LAN. Marginal tightness gives compact sets with outside probabilities below ε/2; enlarge them to closed balls. Their product is closed and bounded in Rk+d and hence compact by [F11]. By [F12] the joint outside probability is at most ε, so the joint laws of (Tn,Δn) are tight.

2.1A1F1F4F7F9step 1.1given

By [F4], choose a subsequence along which (Tn,Δn)⇒(T,Δ). Its marginals have laws W and Nd(0,K). Fix h∈Rd. By [F1], Pn,0(Ln,h=0)→0; define ℓn,h=log⁡Ln,h when Ln,h>0 and ℓn,h=0 otherwise. This replacement changes a variable only on an event of probability o(1). For any a∈Rk and b∈R, the projection of (Tn,ℓn,h) differs by oP(1) from aTTn+bhTΔn−12bhTKh. The latter converges by [F7]; applying scalar Slutsky and [F9] to every projection gives (Tn,ℓn,h)⇒(T,hTΔ−12hTKh).

3.1F1F2F3F7F9step 2.1given

By the continuous mapping theorem [F7], (Tn,eℓn,h)⇒(T,ehTΔ−12hTKh). Since eℓn,h=Ln,h off the event Ln,h=0, whose probability tends to zero by [F1], the same joint limit holds with Ln,h. Apply [F3] to this mean-one proxy; [F1] transfers its limit under the proxy law to the actual Pn,h law. Regularity and δn(h)→Dh imply that under this same actual law Tn⇒W+Dh: apply [F7] to each real projection and then [F9]. Therefore for every real h and t∈Rk, E[eitTT+hTΔ−12hTKh]=eitTDhφW(t), where φW(t)=E[eitTW].

4.1F5F6step 3.1given

Rearranging step 3.1 gives Ft(h):=E[eitTT+hTΔ]=e12hTKh+itTDhφW(t) for every real h. For complex z∈Cd, define Ft(z)=E[eitTT+zTΔ]. On every compact set of z, its coordinate power series and derivatives are dominated by eR∥Δ∥1 for some finite R, whose expectation is finite because Δ is Gaussian. Thus Ft is entire in each coordinate, as is the right-hand side. Apply [F5] one coordinate at a time, holding the remaining coordinates real and then extending them successively; the equality holds for every complex z.

5.1F6F13step 4.1givenalgebra

Put z=iu in step 4.1. By [F13], the resulting characteristic functions satisfy, for all real t,u, E[eitTT+iuTΔ]=φW(t)e−tTDu−uTKu/2. Set R=T−DK−1Δ and replace u by u−K−1DTt. Direct expansion gives E[eitTR+iuTΔ]=(φW(t)etTDK−1DTt/2)e−uTKu/2. At u=0 the first factor is the characteristic function of R; the second is the characteristic function of Δ. The joint characteristic function therefore factors as the product of the marginal characteristic functions.

6.1A1F6F8F9F10F13F14step 5.1algebra∎

Let ρ=L(R)⊗Nd(0,K), whose product law exists by [F10]. For each t∈Rk and u∈Rd, the function (r,z)↦eitTreiuTz is bounded and hence integrable under ρ; [F14] factors its integral into the marginal characteristic functions. By [F13] and step 5.1, this product equals the characteristic function of (R,Δ). Scalar uniqueness [F8] and [F9] give L(R,Δ)=ρ, so R and Δ are independent. Since DK−1Δ∼Nk(0,DK−1DT) and T=DK−1Δ+R, the law of T, which is W, is the stated convolution. The argument allows singular DK−1DT and uses no inverse of that matrix.

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