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Spencer's bound from the partial colouring step and the residual colouring

Proved
Komlos.spencer_recursion

by Hartmann_Psi · Sep 8, 2026 · Mathlib 0df444a (Lean v4.33.1)

Assume both of the following.

  1. (Partial colouring step.) For every 0/10/10/1 matrix, every nonempty active set TTT of columns and all parameters 0<θ≤1/20<\theta\le 1/20<θ≤1/2, ν≥2\nu\ge2ν≥2 obeying n⋅2e−ν2/2(34ν2+2)≤23θ2∣T∣n\cdot 2e^{-\nu^{2}/2}(\tfrac34\nu^{2}+2)\le\tfrac{2}{3}\theta^{2}|T|n⋅2e−ν2/2(43​ν2+2)≤32​θ2∣T∣, there is a partial colouring of TTT leaving at most a θ\thetaθ fraction of TTT uncoloured whose row sums are at most ν∣T∣\nu\sqrt{|T|}ν∣T∣​ in absolute value.
  2. (Residual colouring.) Every set TTT of columns carries a ±1\pm1±1 colouring whose row sums are at most 2∣T∣log⁡(4n)\sqrt{2|T|\log(4n)}2∣T∣log(4n)​ in absolute value.

Then every n×nn\times nn×n matrix with entries in {0,1}\{0,1\}{0,1} admits signs εj∈{±1}\varepsilon_j\in\{\pm1\}εj​∈{±1} with

∣∑j=1nAijεj∣  ≤  6nfor every i,\Bigl|\sum_{j=1}^{n}A_{ij}\varepsilon_j\Bigr|\;\le\;6\sqrt{n}\qquad\text{for every }i,​j=1∑n​Aij​εj​​≤6n​for every i,

which is Spencer's theorem. What has to be supplied is the scheduling: the fraction colored at each application, the parameter ν\nuν used there, the point at which the recursion is stopped in favour of the residual colouring, and the verification that the resulting geometric series of costs stays below 666. The constant 666 is the one from the original paper, and it is what makes the schedule delicate: a random colouring alone gives Θ(nlog⁡n)\Theta(\sqrt{n\log n})Θ(nlogn​), and the recursion has to pay for every round.

Formalization Note The two inputs appear as explicit hypotheses so that this statement is exactly the combinatorial bookkeeping of the argument and nothing else; they are the separately stated partial colouring and residual colouring problems. Colourings are encoded as real-valued vectors, with 000 marking an uncoloured coordinate in the partial colourings.

Preamble
import Mathlib
import Definitions.Def_Komlos_model
open Finset
Formal statement
namespace Komlos

theorem spencer_recursion
    (pcl : ∀ (n : ℕ) (A : Fin n → Fin n → ℝ), (∀ i j, A i j = 0 ∨ A i j = 1) →
      ∀ (T : Finset (Fin n)), 0 < T.card → ∀ θ ν : ℝ, 0 < θ → θ ≤ 1 / 2 → 2 ≤ ν →
        (n : ℝ) * (2 * Real.exp (-(ν ^ 2) / 2) * (3 * ν ^ 2 / 4 + 2))
            ≤ (2 / 3) * θ ^ 2 * (T.card : ℝ) →
        ∃ χ : Fin n → ℝ,
          (∀ j, χ j = 1 ∨ χ j = -1 ∨ χ j = 0) ∧
          (∀ j, j ∉ T → χ j = 0) ∧
          (1 - θ) * (T.card : ℝ) ≤ ((T.filter (fun j => χ j ≠ 0)).card : ℝ) ∧
          (∀ i, |∑ j ∈ T, A i j * χ j| ≤ ν * Real.sqrt (T.card : ℝ)))
    (fin : ∀ (n : ℕ), 0 < n → ∀ (A : Fin n → Fin n → ℝ), (∀ i j, A i j = 0 ∨ A i j = 1) →
      ∀ (T : Finset (Fin n)),
        ∃ χ : Fin n → ℝ,
          (∀ j, j ∈ T → (χ j = 1 ∨ χ j = -1)) ∧
          (∀ j, j ∉ T → χ j = 0) ∧
          (∀ i, |∑ j ∈ T, A i j * χ j|
              ≤ Real.sqrt (2 * (T.card : ℝ) * Real.log (4 * n))))
    (n : ℕ) (A : Fin n → Fin n → ℝ) (h01 : ∀ i j, A i j = 0 ∨ A i j = 1) :
    ∃ ε : Fin n → ℝ, IsSignVector ε ∧
      ∀ i, |∑ j, A i j * ε j| ≤ 6 * Real.sqrt n := by sorry

end Komlos
Source
J. Spencer, Six standard deviations suffice, Trans. Amer. Math. Soc. 289 (1985) 679-706, Theorem 1 and Section 2 (the entropy / partial colouring method), https://doi.org/10.1090/S0002-9947-1985-0784009-0

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