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Synchronization for sufficiently large logarithmic density constants

PDF p. 31: Paragraph after equation (6.2)

Statement

Assume countable choice. For every fixed 0<ε<1, there is γ0=γ0(ε)>1 such that, uniformly for p∈[γ0log⁡n/n,1], with probability tending to one as n→∞, the simple graph G(n,p) is an (n,d∗,α,c−,c+)-expander with d∗=np,α=(2+ε)(np)−1/2,c−=−ε,c+=ε, and satisfies the hypotheses of Theorem 1.10; hence it is globally synchronizing. Uniformity means the supremum of the failure probability over the stated p interval tends to zero; it includes every varying sequence p=p(n) in that range.

Facts & Assumptions

[F2]

The uniform norm estimate with constant 2+ε is Uniform constant-two adjacency control above a large logarithmic density. The simultaneous degree slack and numerical condition for K=2+ε are Uniform degree slack and the vanishing synchrony condition.

[F3]

Degree slack and the adjacency norm imply the two-sided expander by Degree control implies two-sided expansion — Proposition 3.2. Its positive-reference synchrony criterion is General two-sided spectral criterion — Theorem 1.10, under The Axiom of Countable Choice (AC_ω).

[F4]

For any finite family of events in one probability space, the probability of their union is at most the sum of their probabilities (The finite union bound).

Proof

Given: The objects and hypotheses in the Statement.

Goal: Prove all stated conclusions.

1.1F1F2F3F4givenalgebra

Choose γ0>max⁡{1,gε,12/ε2}, where gε is supplied by [F2]. The uniform spectral estimate persists on the smaller parameter interval with this larger lower endpoint. Intersect it with the degree good event from [F2]. By [F4] applied to the two complementary bad events, the failure probability is at most the sum of their two uniform bounds, tending to zero. On this intersection, ∥A−pJ∥≤(2+ε)np=αd∗ and the exact degree inequalities of Proposition 3.2 hold. Thus [F3] establishes all three expander inequalities in the conventions of [F1].

2.1F1F2F3step 1.1algebra∎

Since 0<ε<1, c−>−1. Supplier [F2] gives d∗>0, 0<α≤1/5 and the strict numerical condition of Theorem 1.10 uniformly for large n, because 1+4c+−3c−=1+7ε. Apply [F3] under countable choice. Each graph on the good event is globally synchronizing. The probability conclusion, including every varying allowed sequence p(n), follows from step 1.1 and the uniform quantifiers in [F1].

Source notes

The original large-density claim on PDF p. 31, after (6.2), is retained: for sufficiently large γ0, γ≥γ0 gives d∗=pn, α=(2+ε)(pn)−1/2, c−=−ε, c+=ε, the Theorem 1.10 condition and global synchrony with high probability for p≥γlog⁡n/n. The explicit range 0<ε<1 spells out its fixed small-parameter convention. The local proof preserves its rich parameter conclusion and makes the probability uniformity precise. The local proof uses Uniform constant-two adjacency control above a large logarithmic density and Uniform degree slack and the vanishing synchrony condition. The article and the fixed-law Péché–Soshnikov result are not logical suppliers for the spectral estimate.

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