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Logarithmic L-r covering bound for VC classes of sets

Statement

Let C be a nonempty class of measurable subsets of a measurable space, with finite VC index V as in Shattering, VC dimension, and first-unshattered index for set classes. Let Q be a probability measure, 1≤r<∞, and 0<δ<1. Put K:=3e2/(e−1). Then

N(δ,C,Lr(Q))≤[Kδ−rlog⁡(3eδr)]V−1.

For sets, ∥1C−1D∥Q,r=Q(C△D)1/r. In the case V=1, the exponent is zero and the class consists of one set modulo equality, so the bound is N=1.

Facts & Assumptions

Given: A nonempty measurable set class C with finite VC index V, a probability measure Q, 1≤r<∞, and 0<δ<1.

[F1]

For a set class, the Lr(Q) distance is Q(C△D)1/r; N is the covering number with centers in the class, and D uses strict pairwise separation (Covering, packing, bracketing numbers, and metric entropy).

[F2]

A nonempty class has VC dimension d=V−1 when V is its least positive unshattered size; traces on arbitrary finite ground sets define shattering (Shattering, VC dimension, and first-unshattered index for set classes).

[F3]

A family of subsets of [n] with VC dimension at most d has size at most ∑j=0d(nj) (Sauer–Shelah: a family on [n] of VC dimension at most d has at most ∑i=0d(ni) members).

[F4]

The finite product probability Q⊗n exists and assigns a measurable rectangle A1×⋯×An mass ∏iQ(Ai) (The product sigma-algebra and its finite iterates, For sigma-finite factors, the product measure exists, has the rectangle formula, is sigma-finite, and is unique).

[F5]

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

[F6]

For 0≤u≤1 and a natural n, (1−u)n≤e−nu, and 1+x≤ex for real x (1+x≤exp⁡(x) for every real x, hence (1−p)m≤exp⁡(−mp)).

[F8]

N(ϵ,C,d)≤D(ϵ,C,d)≤N(ϵ/2,C,d) with strict separation and closed-ball radii as stated (Packing and covering numbers with explicit radius conventions).

[F11]

Natural powers obey their recursive definition and are monotone in nonnegative bases (Integer powers am, Monotonicity of x↦xn and of n↦an).

[F12]

For a>0, ar=exp⁡(rlog⁡a) and positive-base real powers obey their product and iterated-power laws (Real powers for positive bases, with the zero-base positive-exponent convention, The exponent, product, quotient, and iterated-power laws for positive real bases and real exponents).

[F13]

For every r>0, t↦tr is strictly increasing on [0,∞) (Positive real powers preserve measurability).

[F14]

The inequality 1+x≤ex for x≥0 gives e≥2 by taking x=1 (1+x≤exp⁡(x) for every real x, hence (1−p)m≤exp⁡(−mp)).

[F15]

For a real s, its integer part k satisfies k≤s<k+1, so n:=k+1 obeys s<n≤s+1 (Integer part: for every real x there is exactly one integer m with m≤x<m+1).

Proof

Given: The class, measure, index, and radii in the statement.

1.1F2given

Put d:=V−1. If d=0 and two sets C,D∈C differed at a point x, their traces on {x} would be both ∅ and {x}, so {x} would be shattered, contradicting [F2]. Thus all class members are equal, N(δ,C,Lr(Q))=1, and the bound holds.

1.2F1F4F15given

Suppose d≥1. Fix any finite subfamily C1,…,Cm with pairwise Lr(Q) distances greater than δ, where m≥2, and set η:=δr∈(0,1) and s:=2log⁡(m)/η>0. Let k:=⌊s⌋ from [F15] and n:=k+1; then s<n≤s+1. Sample X1,…,Xn independently from Q on the finite product probability space supplied by [F4].

1.3F9F10algebra

For u≥1, define h(u):=u/e−log⁡u. By [F10], h′(u)=1/e−1/u, so the mean value theorem makes h decreasing on [1,e] and increasing on [e,∞). Since h(e)=0 by [F9], log⁡u≤u/e for every u≥1.

2.1F1F4F5F6F13step 1.2algebra

For j≠k, the probability that Cj and Ck have identical traces on this sample is [1−Q(Cj△Ck)]n≤(1−η)n≤e−nη: strict separation and [F13] give Q(Cj△Ck)>δr=η, and then [F4] and [F6] apply. The union bound [F5] makes the probability of any collision at most (m2)e−nη≤(m2)m−2<1.

2.2F9F14givenalgebrastep 1.2

Define B:=Kη−1log⁡(3e/η). By [F9], K=3e2/(e−1)>e; by [F14], e≥2; also η−1>1 and log⁡(3e/η)>log⁡e=1. Hence B>e≥2.

3.1F2F3step 2.1choose

Consequently some sample has m distinct traces on [n]. The induced trace family cannot shatter a set of indices containing a repeated sample value, because those two coordinates always have identical membership. Any shattered set of distinct sampled values is shattered by C, so [F2] bounds its size by d. Applying [F3] to the induced family gives m≤∑j=0d(nj).

3.2F9F11step 2.2algebrastep 1.2

If log⁡m≤d, then m≤ed<Bd by step 2.2 and monotonicity of the exponential.

4.1F6F7F11step 3.1algebrastep 2.2

If d≥n, [F7], [F11], and step 2.2 give ∑j=0d(nj)=2n≤2d<Bd, so step 3.1 gives m<Bd. If d<n, put x:=d/n∈(0,1). For j≤d, the recursion xk+1=xkx and 0<x<1 show successively that xd≤xj. Thus [F7] and [F6] give xd∑j=0d(nj)≤∑j=0d(nj)xj≤(1+x)n≤enx=ed, hence ∑j=0d(nj)≤(en/d)d. By step 3.1, m≤(en/d)d.

5.1F9F12step 3.1step 4.1algebrastep 1.2

Suppose log⁡m>d. Then n≥2log⁡(m)/η>2d, so d<n, and log⁡(m)/η>1 gives n≤3log⁡(m)/η. Combining steps 3.1 and 4.1 yields m≤(3elog⁡(m)/(dη))d. Set y:=3e/η and z:=exp⁡((log⁡m)/d)>e. By [F9] and [F12], zd=m and log⁡z=(log⁡m)/d, hence z/log⁡z≤y.

6.1F9F11step 5.1step 1.3algebra

In step 5.1, z>e implies u:=log⁡z>1. From z≤ylog⁡z, monotonicity and the product law in [F9] give u≤log⁡y+log⁡u≤log⁡y+u/e by step 1.3. Thus u≤ee−1log⁡y, and step 5.1 yields z≤ee−1ylog⁡y=Kη−1log⁡(3e/η)=B. Therefore m=zd≤Bd by [F11].

7.1F8step 1.1step 2.2step 4.1step 3.2step 5.1step 6.1∎

If d=0, step 1.1 proves the result. If d≥1, step 2.2 gives B>1, so one-member separated families satisfy the bound, and steps 4.1, 3.2, 5.1 and 6.1 bound every larger finite δ-separated subfamily by Bd. Thus the packing number is finite and its largest integer size is attained; [F8] then gives N(δ,C,Lr(Q))≤D(δ,C,Lr(Q))≤Bd. Substituting d=V−1 proves the statement.

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