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AlgorithmAlgorithm: Literature-sourcedProof: AI-adaptedPipeline-generatedjudge pass (gpt-6-luna)
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  • Literature-sourced: the exact statement appears in a cited source; only wording and notation differ.
  • AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
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Finite-class empirical-risk selection with exponential weights

Definition

Let (X,ΣX) be a nonempty measurable space, let the binary label set {0,1} carry its power-set sigma-algebra, and let C be a finite nonempty class of measurable maps h:X→{0,1}. Fix an integer n≥1 and a parameter ε>0. On an ordered labeled sample S=((x1,y1),…,(xm,ym)) with m≥n, or on its underlying finite multiset, define L^S(h)=1m∑i=1m1{h(xi)≠yi},wS(h)=exp⁡ ⁣(−εn2L^S(h)). Output h∈C with probability wS(h)/∑g∈CwS(g). This formula is invariant under permutations of the sample. For the statistical input, the ordered sample space has its finite product sigma-algebra. For the private-database input, the same formula is defined on finite-support record-count functions of size at least n, with the power-set sigma-algebra; privacy is proved separately.

Definition references

Finite product sigma-algebras for samples

The real exponential function

Probability kernels for randomized selection

Cynthia Dwork and Aaron Roth, The Algorithmic Foundations of Differential Privacy, Definition 3.4 and Chapter 11 §11.1: full text.

Salil Vadhan, The Complexity of Differential Privacy, Proposition 4.2: full text.

Facts & Assumptions

Given: The measurable binary class, n≥1, ε>0, and a finite sample or multiset with at least n records.

[F1]

A measurable space is a set with a sigma-algebra; measurable rectangles generate the finite product sigma-algebra. Measurable spaces and measurable sets, Measurable rectangles in a product of measurable spaces, The product sigma-algebra and its finite iterates

[F3]

The exponential is continuous, continuous real maps have Borel preimages, and composition with a Borel measurable outer map preserves measurability. The exponential function is strictly increasing, A continuous map has Borel preimages of Borel sets, Composition with a Borel measurable outer map preserves measurability

[F4]

Finite sums, scalar multiples, and products of measurable real functions are measurable. Arithmetic and lattice operations preserve measurability whenever they are defined

[F5]

A real function is measurable exactly when its strict threshold sets are measurable. Threshold characterisations of real-valued and extended-real-valued measurability

[F6]

A probability measure has total mass one; a probability kernel has probability-measure sections and measurable event evaluations. Probability measures and probability spaces, Measure kernel and probability kernel

Verification

Given: The hypotheses and procedure in the Definition.

1.1givenF1F2F3F4construct

For each h∈C, its mismatch set is (h−1{0}×{1})∪(h−1{1}×{0}), a finite union of measurable rectangles by [F1]. Thus each sample loss indicator and the finite average L^S(h) are measurable in the ordered sample. By [F2]–[F4], the weight wS(h) is finite, positive, and measurable in S.

2.1F2F4F5F6step 1.1algebra∎

Since C is finite and nonempty, ZS:=∑g∈CwS(g) is finite, measurable, and strictly positive. For a≤0, {S:1/ZS>a} is the whole sample space; for a>0, it is {S:ZS<1/a}, which is measurable by [F5]. Hence 1/ZS, and then each pS(h):=wS(h)/ZS, is measurable by [F4]. These probabilities sum to one. Define K(S,A)=∑h∈ApS(h) for A⊆C; it is countably additive because a disjoint family of subsets of finite C has only finitely many nonempty members, and K(S,C)=1. For each A, S↦K(S,A) is measurable as a finite sum of measurable functions; on the power-set private-database domain every event evaluation is also measurable. Thus [F6] gives the probability-kernel conditions.

Source notes

The scale 1/n is used as a sensitivity upper bound. The private theorem proves its add/remove validity on the restricted size-at-least-n domain and derives privacy and utility directly; no exact attained-maximum convention is assumed here.

Depends on

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Sources