The Häggkvist–Janssen bound on the list chromatic index of complete graphs
A Lean 4 / Mathlib formalization of the Häggkvist–Janssen theorem χ′ₗ(Kₙ) ≤ n: for arbitrary lists of at least n colours on the edges of the complete graph on n vertices (from an arbitrary colour type), there is a proper edge colouring, i.e. a colouring of Mathlib's line graph of ⊤, choosing every colour from the list of its edge. Orders 0 and 1 are included. The proof constructs an explicit local-rank certificate, shows by a forced-choice induction that the corresponding coefficient of an extended-star polynomial is nonzero, and applies Mathlib's combinatorial Nullstellensatz; it is an…
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