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Coordinate Poincare inequalities bound total Sobol effects

Statement

Assume Countable Choice and Dependent Choice. Let X=(X1,…,Xd) have a finite product probability law, let f be a real class in L2, and choose a real product-measurable representative F of f. Fix i such that its marginal law is μi(A)=λ(A∩[ai,bi])/Li with Li=bi−ai>0. Assume that for almost every fixed x−i, the section xi↦F(xi,x−i) is absolutely continuous and its derivative ∂iF is square-integrable under the product law. If V=Var⁡(f(X))>0, then STi≤Li2π2 E∣∂iF(X)∣2V. More generally, the same conclusion holds with Li2/π2 replaced by any constant Ci for which each such almost-everywhere coordinate section of F satisfies Var⁡Xi(g)≤CiEXi∣g′∣2.

Facts & Assumptions

Given: Countable Choice and Dependent Choice, the product law, a real product-measurable representative F of a real f∈L2, the regularity assumptions in the Statement, and positive output variance.

[A1]

The Axiom of Countable Choice (ACω): every sequence of nonempty sets has a choice function.

[A2]

The axiom of dependent choice: a relation in which every element is related to something admits an N-indexed chain: for a nonempty set X, an entire relation R on X, and a∈X, there is x:N→X with x0=a and xnRxn+1 for every n.

[F1]

Independent product inputs have an orthogonal Hoeffding decomposition: f is the orthogonal sum of its product ANOVA components, each fu depends only on Xu, and Pu∖{j}fu=0 for every j∈u.

[F2]

First-order and total Sobol indices: VSTi=∑u∋iVu when V>0.

[F3]

Sharp Poincare bound for the uniform law on an interval: for the uniform law on an interval of length Li, the coordinate variance of an absolutely continuous section is at most (Li2/π2) times its derivative's second moment.

[F4]

Fubini's theorem for L^1 functions on a sigma-finite product and Tonelli's theorem for nonnegative measurable functions on a sigma-finite product: the almost-everywhere coordinate inequalities may be integrated over the remaining product coordinates.

[F5]

Product-coordinate averaging operators: P−i averages only coordinate i and lifts the section average to the full product space.

[F6]

Product-coordinate projections commute and contract in L2: PJPK=PJ∩K for every pair of coordinate subsets.

Proof

Given: The hypotheses, axioms [A1]–[A2], and facts [F1]–[F6].

1.1F1F2F5F6

For i∉u, [F5] and dependence only on Xu give P−ifu=fu. For i∈u, dependence gives Pufu=fu, so [F6] yields P−ifu=P−iPufu=Pu∖{i}fu=0 by the coordinate-centering property in [F1]. Linearity and reconstruction in [F1] therefore give f−P−if=∑u∋ifu. Orthogonality [F1] and the index definition [F2] give ∥f−P−if∥22=∑u∋iVu=VSTi.

2.1A1A2F3F4F5step 1.1

For almost every x−i, apply [F3] to the section of the chosen representative F. Its variance is ∫∣F−P−iF∣2dμi, and its derivative moment is ∫∣∂iF∣2dμi. Integrate the inequality over x−i; [F4] justifies the iterated integrals and gives ∥f−P−if∥22≤(Li2/π2)E∣∂iF(X)∣2.

3.1F2step 1.1step 2.1givenalgebra∎

Substitute the identity from step 1.1 into step 2.1 and divide by V>0. This proves the displayed bound. The same calculation with any valid coordinate constant Ci proves the stated general form.

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