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Rademacher uniform generalization bounds

Statement

Assume the Axiom of Choice (AC). Let (Z,A,P) be a probability space, let m≥1, and let G be a nonempty class of measurable functions g:Z→[0,1]. Take S=(Z1,…,Zm) to be the coordinate sample with law Pm. Put Pmg=m−1∑ig(Zi) and G‾:={1−g:g∈G}. Assume the sample, independent-copy and sign suprema over G and G‾ used in the displayed claims and the symmetrization argument are measurable. Let σ1,…,σm be independent uniform signs, and let

R^S(G):=Eσsup⁡g∈G1m∑i=1mσig(Zi),Rm(G):=ES∼PmR^S(G),

where the σi are independent uniform signs, and let δ∈(0,1). Each of the following four guarantees holds with probability at least 1−δ over S:

∀g∈G,Pg≤Pmg+2Rm(G)+log⁡(1/δ)2m,

∀g∈G,Pg≤Pmg+2R^S(G)+3log⁡(2/δ)2m,

sup⁡g∈G∣Pg−Pmg∣≤2Rm(G)+log⁡(2/δ)2m,

sup⁡g∈G∣Pg−Pmg∣≤2R^S(G)+3log⁡(4/δ)2m.

The four guarantees are separate probability statements. The choice axiom is used through the bounded-differences theorem.

Statement references

One-sided Rademacher complexity: fixes the one-sided empirical and expected complexities and their 1/m normalization.

One-sided Rademacher symmetrization bound: bounds the expected one-sided deviation by twice the expected complexity.

McDiarmid bounded-differences inequality: supplies bounded-differences concentration under AC.

The Axiom of Choice: AC is the stated set-theoretic axiom carried by the concentration theorem.

The natural logarithm as the inverse of the exponential function: defines log⁡ as the inverse of the exponential on positive inputs.

Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm: gives the reciprocal law log⁡(1/x)=−log⁡x and monotonicity of log⁡.

Basic identities for a probability measure: gives finite subadditivity for the union bounds.

Mohri et al., Theorem 3.3 and complete proof, printed pp. 31–33: proves both one-sided bounds with the stated constants in the adopted normalization.

Bartlett and Mendelson, Theorems 5 and 8 with proofs, printed pp. 464–467 and 479–480: gives independent risk-bound proofs in its separate absolute/2/m convention.

Facts & Assumptions

Given: AC, IID observations with law P, a nonempty measurable class of [0,1]-valued functions, measurable suprema, m≥1 and 0<δ<1.

[A1]

AC states that every family of nonempty sets has a choice function. It is invoked only through the McDiarmid supplier, whose theorem assumes AC and whose proof uses it in the conditional-expectation / martingale construction. The Axiom of Choice

[F1]

For g∈G, Pg=∫g dP; R^S(G) is the sign-average one-sided supremum with normalization 1/m, and Rm(G)=ESR^S(G). One-sided Rademacher complexity

[F2]

For an IID sample and measurable suprema, ESsup⁡g(Pg−Pmg)≤2Rm(G); this expectation inequality is choice-free. One-sided Rademacher symmetrization bound

[F3]

If a product-measurable integrable function of independent coordinates changes by at most ci when coordinate i changes, AC gives the upper-tail bound P{f−Ef≥t}≤exp⁡(−2t2/∑ici2); applying it to −f gives the lower-tail version. McDiarmid bounded-differences inequality

[F4]

For x>0, log⁡x is the unique real y with exp⁡(y)=x. The natural logarithm as the inverse of the exponential function

[F5]

log⁡ is strictly increasing, log⁡1=0, and log⁡(1/x)=−log⁡x for x>0; it is the inverse of the exponential on positive inputs. Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm

[F6]

IID means the sample coordinates are mutually independent and have the common law P. Identical distribution and IID families

[F7]

A finite union of events has probability at most the sum of their probabilities. Basic identities for a probability measure

Proof

Given: The hypotheses in the Statement.

1.1A1F1F2F3F4F5F6given

Put Ψ(S):=sup⁡g∈G(Pg−Pmg). Changing one coordinate changes each term in this supremum by at most 1/m, hence ∣Ψ(S)−Ψ(S′)∣≤1/m. By the Statement, Ψ is measurable, and it is bounded because 0≤g≤1. Since 0<δ<1, [F5] and m≥1 give t:=log⁡(1/δ)/(2m)>0; [F4]–[F5] give exp⁡(−2mt2)=δ. McDiarmid [F3] with ci=1/m gives Ψ(S)≤EΨ(S)+t with probability at least 1−δ, and [F2] bounds its expectation by 2Rm(G). This proves the first one-sided bound.

1.2F1givenalgebra

For each fixed sample, R^S(G‾)=Eσ[m−1∑iσi+sup⁡g∈Gm−1∑i(−σi)g(Zi)]. The first term has mean zero and −σ has the same uniform law as σ, so this equals R^S(G); taking expectations gives Rm(G‾)=Rm(G).

2.1A1F1F3F4F5F6step 1.1given

For each fixed sign vector, changing one observation changes every m−1∑iσig(Zi) by at most 1/m, since g takes values in [0,1]; the same bound holds for the supremum and then for its sign average R^S(G). This statistic is bounded and measurable by the Statement. With t=log⁡(1/δ)/(2m)>0 as in step 1.1, [F3] applied to its negative gives Rm(G)≤R^S(G)+t with probability at least 1−δ.

2.2F1F7step 1.1step 1.2algebra

For h=1−g, Ph−Pmh=Pmg−Pg. Apply step 1.1 to G and G‾ with failure probability δ/2 each. By [F7] and step 1.2, both deviations are at most 2Rm(G)+log⁡(2/δ)/(2m) except on an event of probability at most δ. This proves the first two-sided bound.

3.1F1F4F5F7step 1.1step 2.1algebra

Apply step 1.1 with failure probability δ/2 and step 2.1 with failure probability δ/2. By [F7], both events hold with probability at least 1−δ. Since 2/δ>1, [F5] gives t:=log⁡(2/δ)/(2m)>0; combining the two bounds gives Ψ(S)≤2Rm(G)+t≤2R^S(G)+3t. This is the second one-sided guarantee.

4.1F1F7step 1.2step 3.1algebra∎

Apply step 3.1 to G and G‾ with failure probability δ/2 each. Their complexities agree by step 1.2, and their upper deviations are sup⁡g(Pg−Pmg) and sup⁡g(Pmg−Pg), respectively. By [F7] the two bounds hold together with probability at least 1−δ, giving the last two-sided bound with log⁡(4/δ).

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