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Atlas.ProbabilisticMethodsInCombinatorics.code.Chapter8.ChromaticNumber

theorem ChromaticRandom.property_star_whp (ε : ℝ) :
0 < ε → ∀ᶠ (n : ℕ) in Filter.atTop, (ChromaticNumber.probGnHalf n fun (G : SimpleGraph (Fin n)) => ∃ (S : Finset (Fin n)), S.card ≥ n / Nat.log 2 n ^ 2 ∧ ∀ I ⊆ S, I.card ≥ ChromaticNumber.k₀ n - 3 → ¬G.IsIndepSet ↑I) ≤ Real.exp (-↑n ^ (1 - ε))

Property $\star$ for $G(n, 1/2)$ holds with high probability: for every $\varepsilon > 0$ and all large $n$, the probability that $G(n,1/2)$ contains a subset $S$ of size $\geq n / (\log_2 n)^2$ none of whose $(k_0 - 3)$-subsets is independent is at most $\exp(-n^{1-\varepsilon})$.

theorem ChromaticRandom.bollobas_chromatic_number (ε : ℝ) :
0 < ε → ∀ᶠ (n : ℕ) in Filter.atTop, (ChromaticNumber.probGnHalf n fun (G : SimpleGraph (Fin n)) => G.Colorable (n / (ChromaticNumber.k₀ n - 3) + n / Nat.log 2 n ^ 2)) ≥ 1 - Real.exp (-↑n ^ (1 - ε))

Bollobás's theorem (Theorem 8.3.2, Bollobás 1988). With high probability, $\chi(G(n, 1/2)) \sim n / (2 \log_2 n)$. Concretely, for every $\varepsilon > 0$ and all large $n$, $G(n, 1/2)$ is colorable with $n/(k_0 - 3) + n/(\log_2 n)^2$ colors with probability at least $1 - \exp(-n^{1-\varepsilon})$.