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Abstract

The ABC conjecture implies many conjectures and theorems in number theory, including the celebrated Fermat's Last Theorem. Mason-Stothers Theorem is a function field analogue of the ABC conjecture that admits a much more elementary proof with many interesting consequences, including a polynomial version of Fermat's Last Theorem. While years of dedicated effort are expected for a full formalization of Fermat's Last Theorem, the simple proof of Mason-Stothers Theorem and its corollaries calls for an immediate formalization. We formalize an elementary proof by Snyder in Lean 4, and also formalize many consequences of Mason-Stothers, including nonsolvability of Fermat-Cartan equations in polynomials, nonparametrizability of a certain elliptic curve, and Davenport's Theorem. We compare our work to existing formalizations of the Mason-Stothers by Eberl in Isabelle and Wagemaker in Lean 3 respectively. Our formalization has been integrated into the mathlib library of Lean 4.