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VC entropy uniform deviation without a logarithmic sample-size factor

Statement

Let (X,Σ) be a nonempty measurable space, let H be a nonempty class of measurable binary classifiers with finite VC dimension d, and put Z:=X×{0,1} with sigma-algebra ΣZ:=Σ⊗P({0,1}). Let P be a probability law on Z, and let S=(Z1,…,Zm) consist of m≥1 IID observations with law P. For h∈H, put gh(x,y):=1{h(x)≠y}, GH:={gh:h∈H}, and G‾H:={1−g:g∈GH}. Define RP(h):=P{h(X)≠Y} and R^S(h):=m−1∑i=1mgh(Zi). For measurable g:Z→[0,1], write Pg:=∫g dP and Pmg(S):=m−1∑i=1mg(Zi). Let S′=(Z1′,…,Zm′) be an independent sample with law Pm, and write Pm′g:=Pmg(S′).

Assume the following maps are measurable:

  1. S↦sup⁡g∈GH(Pg−Pmg(S)) and S↦sup⁡gˉ∈G‾H(Pgˉ−Pmgˉ(S)) on (Zm,ΣZ⊗m);
  2. (S,S′)↦sup⁡g∈GH(Pm′g−Pmg(S)) and (S,S′)↦sup⁡gˉ∈G‾H(Pm′gˉ−Pmgˉ(S)) on (Zm×Zm,ΣZ⊗m⊗ΣZ⊗m);
  3. for every fixed σ∈{−1,+1}m, the maps (S,S′)↦sup⁡g∈GHm−1∑i=1mσi(g(Zi′)−g(Zi)) and (S,S′)↦sup⁡gˉ∈G‾Hm−1∑i=1mσi(gˉ(Zi′)−gˉ(Zi)) on (Zm×Zm,ΣZ⊗m⊗ΣZ⊗m);
  4. for every fixed σ∈{−1,+1}m, the maps S↦sup⁡g∈GHm−1∑i=1mσig(Zi) and S↦sup⁡gˉ∈G‾Hm−1∑i=1mσigˉ(Zi) on (Zm,ΣZ⊗m).

The first pair of maps gives the two one-sided deviation suprema and makes the absolute-deviation supremum in the conclusion measurable. Set K0:=3e2/(e−1), K:=4K0, A:=12eK+2, and CVC:=38A(log⁡A+2log⁡2), where e:=exp⁡(1) and log⁡ is the natural logarithm. If the Axiom of Choice (AC) is assumed, then for every 0<δ<1, with probability at least 1−δ,

sup⁡h∈H∣RP(h)−R^S(h)∣≤2CVCd+1m+log⁡(2/δ)2m.

This is a uniform absolute-deviation bound; it claims neither ERM existence nor a distribution-free lower bound.

Statement references

Mohri et al., Theorem 3.3 and proof: one-sided generalization through ghost-sample symmetrization and bounded differences.

Bartlett and Mendelson, Theorem 5 and Appendix B: an independent Rademacher generalization proof in a different normalization.

Facts & Assumptions

Given: A nonempty finite-VC binary class, an IID sample, measurable deviation suprema, and 0<δ<1.

[F1]

For every fixed sample, the zero-one loss class GH has one-sided empirical Rademacher complexity at most CVC(d+1)/m, with explicit CVC. VC entropy bound for empirical Rademacher complexity

[F2]

Population risk is the integral of the loss and empirical risk is its sample average; for zero-one loss these equal the error probability and empirical error fraction. Population and empirical risk

[F3]

The fixed-class risks are measurable functions of the sample. Population and empirical risks are measurable

[A1]

Assume the Axiom of Choice; the concentration supplier below requires it. The Axiom of Choice

[F5]

Under AC, a measurable integrable function with coordinate oscillations at most ci has upper tail Pr⁡{f−Ef≥t}≤exp⁡(−2t2/∑ici2). McDiarmid bounded-differences inequality

[F6]

Probability is subadditive on a finite union. Basic identities for a probability measure

[F8]

Expectation is linear and monotone for integrable real random variables. Linearity, monotonicity, and the modulus bound for expectation

Proof

Given: The hypotheses in the Statement.

1.1F2F3F4F8givenconstructalgebra

For h∈H, put gh(x,y)=1{h(x)≠y} and define Φ(S)=sup⁡h∈H(RP(h)−R^S(h)). On a product probability space take an independent IID copy S′. Each coordinate of S′ has law P, so product-law marginals and Fubini [F4], together with expectation linearity [F8], give RP(h)=ES′R^S′(h) for every h. Monotonicity of expectation then gives Φ(S)≤ES′sup⁡h(R^S′(h)−R^S(h)). The integrand is bounded and measurable by assumption, hence [F4] allows integration in either sample order. For each fixed sign vector, swapping Zi with Zi′ at exactly its negative coordinates preserves the joint product law, so the expected supremum is unchanged when each difference is multiplied by σi. Subadditivity of the supremum and expectation linearity [F8] bound it by the sum of the two expected one-sided empirical Rademacher suprema, which are equal because σ and −σ have the same law. Thus EΦ(S)≤2ES,σR^S(GH).

2.1A1F1F5F7step 1.1algebra

By [F1], EΦ(S)≤2CVC(d+1)/m. Replacing one observation in S changes each R^S(h) by at most 1/m, so the supremum Φ(S) changes by at most 1/m as well. Applying [F5] with ci=1/m and t=log⁡(2/δ)/(2m) gives Pr⁡{Φ(S)>EΦ(S)+t}≤δ/2.

3.1A1F1F4F6F7step 1.1step 2.1algebra∎

The complemented class Hˉ={1−h:h∈H} has the same trace counts and VC dimension as H, and its loss satisfies g1−h(x,y)=1−gh(x,y). Applying steps 1.1–2.1 to Hˉ with the same δ and t therefore gives, except on an event of probability at most δ/2, R^S(h)−RP(h)≤2CVC(d+1)/m+t for every h∈H. The finite union bound [F6] combines this lower-deviation event with step 2.1, giving the asserted absolute-deviation bound with probability at least 1−δ.

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