The Alon–Tarsi theorem and 3-choosability of planar bipartite graphs
A Lean 4 / Mathlib formalization of the Alon–Tarsi theorem: if a finite simple graph has an orientation whose numbers of even and odd Eulerian subdigraphs differ, then it can be properly coloured from any lists larger than the out-degrees; and of its corollary that every finite planar bipartite graph is 3-choosable. Planarity is the combinatorial notion of a plane map (a rotation system of genus zero) of the registered entry PALOMAR-2026-10-15-000005, whose definitions are reproduced verbatim in the Challenge. The proof uses the graph polynomial and Mathlib's Combinatorial Nullstellensatz,…
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