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One-sided VC uniform convergence and the finite-class branch

Statement

Let (X,Σ) be a nonempty measurable space, let H be a nonempty class of measurable binary functions on X with finite VC dimension d, put Z=X×{0,1} with its product sigma-algebra, and let P be a probability law on Z. For h∈H, define RP(h):=P{(x,y):h(x)≠y} and R^S(h):=m−1∑i=1m1{h(Xi)≠Yi} for an IID sample S=((Xi,Yi))i=1m of size m≥1 with law P. Assume every displayed supremum over H and its binary loss class that appears below is measurable. For 0<δ<1, the following separate branches hold.

  1. If H is finite, then without an Axiom of Choice premise, with probability at least 1−δ, sup⁡h∈H∣RP(h)−R^S(h)∣≤log⁡∣H∣+log⁡(2/δ)2m. In particular, if d=0, then H is a singleton and this gives sup⁡h∈H∣RP(h)−R^S(h)∣≤log⁡(2/δ)/(2m).

  2. If 1≤d≤m and the Axiom of Choice (AC) is assumed, then with probability at least 1−δ, simultaneously for every h∈H, RP(h)≤R^S(h)+2dlog⁡(em/d)m+log⁡(1/δ)2m.

Here log⁡ is the natural logarithm and e=exp⁡(1).

Statement references

Mohri et al., Theorem 3.3, Theorem 3.5, Theorem 3.7, Corollary 3.8, and Corollary 3.19: one-sided Rademacher generalization, the finite-sample maximum bound, and the stated VC display.

Bartlett and Mendelson, Theorem 5 and Appendix B: an independent classification-risk bound in their 2/m absolute-supremum normalization.

Facts & Assumptions

Given: A measurable binary class H with finite d=VCdim⁡(H), an IID sample of size m≥1 with law P, measurable deviation suprema, and 0<δ<1.

[F1]

Shattering is defined on indexed tuples; repeated points remain distinct coordinates and d is the supremum of shattered lengths. Indexed shattering and VC dimension of a binary class

[F2]

ΠH(m) is the supremum of trace cardinalities on indexed m-tuples. Growth function of a binary hypothesis class

[F3]

For every integer m≥0, ΠH(m)≤∑j=0d(mj), and for 1≤d≤m it is at most (em/d)d. Sauer–Shelah growth bound for indexed samples

[F4]

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

[F5]

For each fixed measurable classifier, its population and empirical risks are measurable functions of the sample. Population and empirical risks are measurable

[F6]

IID observations are independent and have the same law; their finite product law exists and has the rectangle formula; bounded measurable functions on a product probability space may be integrated in either order. Identical distribution and IID families, For sigma-finite factors, the product measure exists, has the rectangle formula, is sigma-finite, and is unique, Independent random elements have product joint law, Fubini's theorem for L^1 functions on a sigma-finite product

[A1]

Assume the Axiom of Choice for the VC branch. The supplied McDiarmid inequality assumes AC through its conditional-expectation proof. The Axiom of Choice

[F7]

Under AC, if a measurable integrable function on a finite product probability space changes by at most ci when coordinate i alone changes, then its upper tail is at most exp⁡(−2t2/∑ici2). McDiarmid bounded-differences inequality

[F8]

For a finite nonempty class G of measurable [0,1] functions, Pr⁡{sup⁡g∈G∣Pmg−Pg∣≥t}≤2∣G∣e−2mt2, and this finite-class proof is choice-free. Uniform deviation bound for a finite unit-range class

[F9]

If 0≤Y≤1 almost surely, then Eeλ(Y−EY)≤eλ2/8 for every real λ. The Hoeffding MGF bound for a variable in a unit interval

[F11]

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

[F12]

For all x,y∈R and 0≤t≤1, exp⁡((1−t)x+ty)≤(1−t)exp⁡(x)+texp⁡(y). The two-point convexity inequality for the exponential function

Proof

Given: The objects and assumptions of the Statement.

1.1F4F5givenconstruct

Put gh(x,y):=1{h(x)≠y}. Its unit-level set is (h−1{0}×{1})∪(h−1{1}×{0}), so each gh is measurable and takes values in [0,1]. By [F4]–[F5], Pgh=RP(h) and Pmgh=R^S(h) are well-defined, measurable risk functions; the assumed supremum measurability applies to their deviations.

2.1F6F11givenstep 1.1

For an independent IID copy S′ of S, every coordinate of S′ has law P; the product-law and Fubini facts in [F6], together with finite expectation linearity [F11], give RP(h)=ES′Pm′gh for every h. Thus Φ(S):=sup⁡h∈H(RP(h)−Pmgh)≤ES′sup⁡h∈H(Pm′gh−Pmgh). The integrand is bounded and measurable, so product-law invariance and Fubini [F6] allow integration over S,S′. For each fixed sign vector σ, swapping Zi with Zi′ whenever σi=−1 preserves their joint product law; averaging over the finite sign space therefore leaves this expectation unchanged after inserting σi into each difference.

3.1F6F11step 2.1algebra

Subadditivity of the supremum and expectation linearity [F11] give EΦ(S)≤2ES,σR^S(GH), where R^S(GH)=Eσsup⁡hm−1∑iσigh(Zi). Writing h~=2h−1 and Y~i=2Yi−1 gives gh(Zi)=(1−h~(Xi)Y~i)/2; the sign-only term has mean zero and −σiY~i are again independent uniform signs, so R^S(GH)=12Eσsup⁡hm−1∑iσih~(Xi).

4.1F2F9F10F12step 2.1step 3.1algebra

For a fixed sample, the set VS={(h~(X1),…,h~(Xm)):h∈H} has ∣VS∣≤ΠH(m) and each vector has Euclidean norm m. For a fixed v∈{−1,+1}m, Yi=(1+σivi)/2 is in [0,1] with mean 1/2; applying [F9] at parameter 2λ gives Eeλσivi≤eλ2/2. Expanding the finite uniform sign sum factors over coordinates, so Eσeλ∑iσivi≤emλ2/2. For a finite V of size N, eλmax⁡v∑iσivi≤∑veλ∑iσivi. On the finite sign space, the two-point convexity inequality [F12] gives eλEσmax⁡v∑iσivi≤Eσeλmax⁡v∑iσivi; its equal-weight finite form follows by induction over its q=2m outcomes, applying [F12] to the mean of the first q−1 outcomes and the last outcome with weights (q−1)/q and 1/q. Taking logarithms using [F10] yields Eσmax⁡v∈Vm−1∑iσivi≤log⁡N/(mλ)+λ/2. Optimizing λ>0 gives 2log⁡N/m (and the bound is 0 if N=1). By steps 2.1–3.1, EΦ(S)≤2log⁡ΠH(m)/m.

5.1A1F3F7step 4.1algebra

By [F3], when 1≤d≤m the preceding expectation is at most 2dlog⁡(em/d)/m. Changing one sample coordinate changes Φ(S) by at most 1/m, because every gh is [0,1]-valued. Apply [F7] with ci=1/m and t=log⁡(1/δ)/(2m); since ∑ici2=1/m, with probability at least 1−δ we have Φ(S)≤EΦ(S)+t, which is the stated VC branch.

6.1F1F4F8F10givenalgebra∎

If d=0, no two distinct functions in H can differ at any point, since that point would be shattered; hence H contains one function. If H is finite, the loss class {gh:h∈H} is finite with size at most ∣H∣. Put t=(log⁡∣H∣+log⁡(2/δ))/(2m); then 2∣H∣e−2mt2=δ by [F10]. Applying [F8] at this t proves the choice-free branch, including the d=0 case.

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