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Every entry is at most 444 in the no-four orbit-matrix case

Proved
Conway99.no_four_all_entries_le_four

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

combinatoricsconway-99orbit-matrixstrongly-regular-graphs

Let CCC be a 9×99\times 99×9 matrix over N\mathbb{N}N satisfying the Conway99 orbit-matrix equations (symmetric, row sums 141414, (C2)ij+cij(C^2)_{ij}+c_{ij}(C2)ij​+cij​ equal to 343434 on the diagonal and 222222 off it), with every diagonal entry in {0,2,4}\{0,2,4\}{0,2,4} and no diagonal entry equal to 444. Then every entry of CCC is at most 444. Indeed each row has diagonal 000 or 222, and the row-profile census gives fiber counts with no 555s in every case, while a direct square-sum argument excludes entries above 555. This makes the entry domains finite and explicit for the trace-101010, no-four branch of Wilbrink's Theorem 5.

Preamble
import Mathlib.Data.Matrix.Basic
import Mathlib.Algebra.BigOperators.Fin
Formal statement
open scoped BigOperators

namespace Conway99

theorem no_four_all_entries_le_four
    (C : Matrix (Fin 9) (Fin 9) ℕ) (hsymm : ∀ i j, C i j = C j i)
    (hrow : ∀ i, ∑ j, C i j = 14)
    (hsq : ∀ i j, (∑ k, C i k * C k j) + C i j = (if i = j then 12 else 0) + 22)
    (hdiag : ∀ i, C i i = 0 ∨ C i i = 2 ∨ C i i = 4)
    (hfour : ∀ i, C i i ≠ 4) :
    ∀ i j, C i j ≤ 4 := by sorry

end Conway99
Source
H. A. Wilbrink, 'On the (99,14,1,2) strongly regular graph', EUT Report 84-WSK-03, 1984, Theorem 5, pp. 350-354, https://pure.tue.nl/ws/files/2449333/256699.pdf ; entry bound for the trace-10, no-four branch.

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