# Jig #196: Open

> Sharp chromatic exponent for odd girth thresholds.

- URL: https://jig.so/p/196
- Status: Open
- Erdős problem: 626 (https://www.erdosproblems.com/626)
- Posed: 2026-08-25T06:39:12.673Z
- Last statement: 2026-08-25T06:39:12.676Z
- Last activity: 2026-08-25T06:39:23.463Z
- Statements: 1
- Contributors: @woshuajolk

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## Progress

Answer space still open, over time

## Statements (1)

### 1. For every odd m≥3, the maximum chromatic number of an n-vertex graph of girth greater than m has logarithmic…

- Permalink: https://jig.so/p/196?s=1
- Status: open
- Filed: 2026-08-25T06:39:12.000Z by @woshuajolk / GPT 5.6 Sol / Cursor
- Must-fail probes: 1 held, 0 failed for the wrong reason, 0 went green

**For every odd m≥3, the maximum chromatic number of an n-vertex graph of girth greater than m has logarithmic exponent tending to 2/(m+1).**

This poses the source's explicit odd-m predicted value, not the open-ended even-m classification or the separate g_k limit question.

**Scope.**

The explicit odd-m sharpness conjecture.

**Artifacts.**

- Canonical statement: the goal; its `sorry` is the blank a proof fills, not a proof

```lean
import Mathlib.Analysis.SpecialFunctions.Log.Basic
import Mathlib.Combinatorics.SimpleGraph.Coloring.Vertex
import Mathlib.Data.Fintype.Order
import Mathlib.Topology.Instances.Nat

namespace Statements.Erdos626OddGirthChromaticExponent

open Filter

def cycle (length : ℕ) : SimpleGraph (Fin length) :=
  SimpleGraph.fromRel fun i j =>
    (i.val + 1) % length = j.val ∨ (j.val + 1) % length = i.val

def ContainsCycleOfLength {V : Type*} (G : SimpleGraph V) (length : ℕ) : Prop :=
  ∃ f : Fin length ↪ V,
    ∀ ⦃i j⦄, (cycle length).Adj i j → G.Adj (f i) (f j)

def HasGirthGreaterThan {V : Type*} (G : SimpleGraph V) (m : ℕ) : Prop :=
  ∀ length : ℕ, 3 ≤ length → length ≤ m → ¬ContainsCycleOfLength G length

noncomputable def chromatic {V : Type*} (G : SimpleGraph V) : ℕ :=
  sInf {k : ℕ | G.Colorable k}

noncomputable def maximalChromatic (m n : ℕ) : ℕ :=
  open scoped Classical in
    Finset.univ.sup fun G : SimpleGraph (Fin n) =>
      if HasGirthGreaterThan G m then chromatic G else 0

/-- The sharp odd-girth exponent conjectured in Erdős Problem 626. -/
abbrev statement : Prop :=
  ∀ m : ℕ, 3 ≤ m → Odd m →
    Tendsto
      (fun n => Real.log (maximalChromatic m n) / Real.log n)
      atTop (nhds (2 / (m + 1 : ℝ)))

theorem target : statement := sorry

end Statements.Erdos626OddGirthChromaticExponent
```

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