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The score-matching integration-by-parts identity

Statement

Assume the Axiom of Countable Choice used by the library's Lebesgue-measure and weak-derivative representative interfaces. Let d≥1, let λd be Lebesgue measure on Rd, and let p:Rd→[0,∞) be a continuously differentiable probability density, so ∫p dλd=1. Write Pp(A):=∫Ap dλd,A∈L(Rd), which is a probability measure, and let Ep[f]:=∫f dPp for integrable f. Define the data score by sp(x):=∇log⁡p(x) when p(x)>0 and set sp(x):=0 when p(x)=0. For an energy model relative to λd with 0<Zθ=∫e−Eθ dλd<∞, assume Eθ∈C1(Rd) and define its model score field by ψθ:=−∇Eθ. Assume each component of ψθ is locally integrable and has locally integrable weak first derivatives: for each i,j, a locally integrable representative gij satisfies ∫Rdψθ,i ∂jφ dλd=−∫Rdgijφ dλd(φ∈Cc∞(Rd)). Here Cc∞(Rd) denotes the smooth compactly supported test functions. The weak derivatives are unique up to Lebesgue-null sets, and define div⁡ψθ:=∑i=1dgii; the integrals below do not depend on the chosen representatives. The C1 convention is that the first derivative exists everywhere and is continuous. For each admissible θ, assume Ep∥sp∥2<∞,Ep∥ψθ∥2<∞,Ep∣div⁡ψθ∣<∞. Then the score-matching objective J(θ):=12Ep∥ψθ−sp∥2 satisfies J(θ)=Ep[12∥ψθ∥2+div⁡ψθ]+Cp,Cp:=12Ep∥sp∥2, where Cp is independent of θ. No pointwise boundary limit at infinity is assumed.

Statement references

Aapo Hyvärinen, Theorem 1 and Appendix A of the full JMLR paper states the score-matching identity under a pointwise boundary condition.

Kevin P. Murphy, Chapter 24, Sections 24.1–24.3.4 defines energy models, the model score and score matching.

Facts & Assumptions

Given: The hypotheses in the Statement, including Countable Choice for the library Lebesgue measure interface and the three stated finite p-integrability conditions.

[A1]

p is a nonnegative C1 probability density; for each admissible parameter, Eθ∈C1, its score field is locally integrable with locally integrable weak first derivatives, and the score and divergence have the stated finite p-moments.

[A2]

The Axiom of Countable Choice (ACω): every countable family of nonempty sets has a choice function; it is used through [F3], [F8], [F10], [F17], and [F19].

[F1]

Energy-based density and partition function: for 0<Zθ<∞, the model density is e−Eθ/Zθ, and Zθ is independent of x.

[F2]

The measure with density f relative to μ: for a nonnegative measurable density f, f dμ is the measure A↦∫Af dμ.

[F4]

Dominated convergence: almost-everywhere convergence under one integrable majorant implies convergence of the corresponding integrals.

[F5]

Probability measures and probability spaces: a measure of total mass one is a probability measure.

[F6]

Continuously differentiable maps, local inverses, and local diffeomorphisms: a map on an open Euclidean set is C1 when it is differentiable everywhere and its derivative entries are continuous.

[F7]

Weak derivative of a locally integrable function: gij is a weak derivative exactly when its signed test-function identity holds for every smooth compactly supported test function.

[F8]

Uniqueness of a weak derivative as an almost-everywhere class: under Countable Choice, any two locally integrable representatives satisfying the same weak-derivative identity agree almost everywhere.

[F9]

A continuous map has Borel preimages of Borel sets: a continuous map has Borel preimages of Borel sets.

[F10]

Assuming countable choice, every Borel subset of Rn is Lebesgue measurable: under Countable Choice, Borel subsets of Rd are Lebesgue measurable.

[F11]

A smooth bump between concentric Euclidean balls: for 0<r<R, a smooth function χ:Rd→[0,1] can equal 1 on B‾r(0) and have support contained in BR(0).

[F14]

Chain sum product and composition rules for Banach derivatives: total derivatives obey the bounded-bilinear product rule.

[F16]

Monotonicity and nonnegative homogeneity of the nonnegative integral: the nonnegative integral is monotone and positively homogeneous.

[F17]

A C^1 diffeomorphism satisfies the change-of-variables formula for nonnegative Lebesgue measurable functions: a C1 diffeomorphism transforms nonnegative Lebesgue integrals by its absolute Jacobian determinant.

[F18]

The mollifier family generated by a unit-mass smooth bump: a unit-mass smooth compactly supported function generates the scaled family ρϵ(x)=ϵ−dρ0(x/ϵ).

[F19]

Convolution with a mollifier is smooth, and derivatives pass under the integral sign: convolution of a locally integrable function with a smooth compactly supported mollifier is smooth.

[F20]

Differentiation under the integral sign: an integrable parameter-independent majorant permits differentiation under the integral sign.

[F21]

Fermat's theorem: an interior differentiable local extremum has zero gradient: a differentiable function at an interior local extremum has zero gradient.

[F22]

The Jacobian matrix of partial derivatives and the gradient in the scalar-valued case: the gradient coordinates of a scalar-valued function are its coordinate partial derivatives.

[F24]

Proof

1.1givenA1A2F1F2F3F5F6F8F9F10F11F12F13F15F16F17F18F23F24

By [F1], the admissible model has density e−Eθ/Zθ, with Zθ independent of x. Since p is C1, [F6, F23] make it continuous; [F9, F10] then make it Lebesgue measurable, so [F2] makes Pp a measure, and [F5] makes it a probability measure. The local increment bound [F23] gives, at each x∈U={p>0}, constants Cx,δx>0 with ∣p(y)−p(x)∣≤Cx∥y−x∥ for ∥y−x∥<δx; taking ∥y−x∥<min⁡{δx,p(x)/(2(Cx+1))} shows p(y)>0, so U is open. On U, [F6, F15, F24] make sp=∇log⁡p continuous. For each open O⊆Rd, its score preimage is the union of the relative-open preimage in U and Uc when 0∈O; hence the zero extension is Borel and Lebesgue measurable by [F10]. By [F8], weak-derivative representatives agree off a Lebesgue-null set; [F2] makes every such set Pp-null, so the divergence integral is representative-independent. Choose χ from [F11] with r=1 and R=2. By [F9, F10], this continuous bump is Lebesgue measurable. Its gradient vanishes outside B2; [F12, F13] make C:=∥∇χ∥∞ finite. The box [−1/(2d),1/(2d))d lies in B1, since its points have norm at most d/(2d)≤1/2, while supp⁡χ⊂B2⊂[−3,3)d and 0≤χ≤1. Thus [F3, F16] give d−d≤M:=∫χ dλd≤6d. Set ρ0=χ/M; positive homogeneity [F16] gives it unit mass. Let ρ0,ε be its scaled family from [F18]; [F18] gives support in B‾2ε and [F17] gives unit mass. For R>0, define χR(x)=χ(x/R). Then χR=1 on B‾R, vanishes outside B2R, and [F24] gives ∥∇χR∥∞≤C/R.

2.1A1F3F4F7F12F13F14F17F19F20F24step 1.1

Fix R and put φ=pχR. The product rule [F14] makes φ a nonnegative C1 function supported in B‾2R. By [F12, F13], φ and its first derivatives are bounded there; the ball lies in a finite-volume box by [F3], so φ is locally integrable. The convolution φε=φ∗ρ0,ε is smooth by [F19] and supported in B‾2R+2 for ε≤1, since supp⁡φ⊆B‾2R and supp⁡ρ0,ε⊆B‾2. By [F17], applied to the nonnegative convolution integrand, φε(x)=∫φ(x−z)ρ0,ε(z) dz. For each coordinate j, [F24] gives the parameter derivative of f(z,t)=φ(x+tej−z)ρ0,ε(z). Its sections are integrable because φ is bounded and the kernel has mass one; its derivative is measurable and bounded by ∥∂jφ∥∞ρ0,ε(z), an integrable majorant. Thus [F20] gives ∂jφε(x)=∫∂jφ(x−z)ρ0,ε(z) dz. The unit mass and shrinking support give, for each x, ∣φε(x)−φ(x)∣≤sup⁡∥z∥≤2ε∣φ(x−z)−φ(x)∣→0, with the same estimate for each first derivative. Since φ and ∇φ are bounded by [F12, F13], the weak-identity integrands are dominated on this common compact support by constants times ∣ψθ,i∣ and ∣gii∣. Local integrability and [F4] pass the identities from φε to φ. Summing over i and expanding ∇(pχR) with [F14] yields ∫χRψθ⋅∇p dλd+∫pψθ⋅∇χR dλd=−∫pχRdiv⁡ψθ dλd.

2.2givenA1F15F21F22F24step 1.1algebra

On {p>0}, [F15, F24] give ∇p=psp. At any point where p=0, nonnegativity makes it an interior local minimum, so [F21, F22] give ∇p=0 there too. Thus ∇p=psp everywhere. Componentwise ∣ψθ⋅sp∣≤∑i∣ψθ,isp,i∣ and 2∣ab∣≤a2+b2 imply ∫∣ψθ⋅∇p∣ dλd≤12Ep(∥ψθ∥2+∥sp∥2)<∞ and ∫p∣div⁡ψθ∣ dλd<∞. Also ∥ψθ∥≤(1+∥ψθ∥2)/2, so the cutoff term from step 1.1 is bounded in absolute value by C2R(1+Ep∥ψθ∥2)→0.

3.1F4step 2.1step 2.2

Let R=n for integers n→∞. Then χn→1 pointwise and ∣χn∣≤1. The first integrand in step 2.1 is dominated by ∣ψθ⋅∇p∣ and the third by p∣div⁡ψθ∣, both integrable by step 2.2. Applying [F4] to both terms and using the vanishing cutoff term from step 2.2 gives ∫ψθ⋅∇p dλd=−∫pdiv⁡ψθ dλd.

4.1givenA1step 2.2step 3.1algebra∎

Expanding the square gives J(θ)=12Ep∥ψθ∥2+12Ep∥sp∥2−∫pψθ⋅sp dλd. Since ∇p=psp by step 2.2, step 3.1 turns the last term into +Ep[div⁡ψθ]; the remaining data-score term is Cp, independent of θ.

Remarks

The cutoff and integration-by-parts argument is choice-free once the Lebesgue measure and weak-derivative classes are available. Countable Choice is used for those two interfaces; no full Axiom of Choice is used. The cutoff estimate supplies the multivariate limit directly and does not rely on pointwise vanishing of pψθ at infinity.

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