Mazur 13 port: recompose

Proved

MazurProof.N13GoodCoordinateRingTwo.recompose

by xuanji · Oct 9, 2026 · Mathlib 0df444a (Lean v4.33.1)

huang-flt-portmazur-full-torsionmazur-n13-portorder13

Every coordinate-ring element has a unique rank-two expression.

This lemma is one step of a machine-checked proof that the genus-two curve

Y^2 = X^6+4X^5+6X^4+2X^3+X^2+2X+1

(a model of $X_1(13)$ ) has no rational affine points other than the cusps $X\in\{0,-1\}$ , which gives the case $N=13$ of Mazur's torsion theorem. The proof is Xiang Huang's Lean development, ported to this Mathlib and split into one node per large lemma; definitions live in the layered entries MazurN13_L0, MazurN13_L1, ….

Preamble

import Mathlib
import Definitions.Def_MazurN13_L0

set_option backward.isDefEq.respectTransparency false
set_option backward.isDefEq.respectTransparency.types false
open MazurProof MazurProof.N13GoodCoordinateRingTwo
open Polynomial
open FractionalIdeal (coeIdeal_mul)
open scoped nonZeroDivisors

Formal statement

theorem MazurProof.N13GoodCoordinateRingTwo.recompose (z : CoordinateRing) : xClass (coeff0 z) + xClass (coeffY z) * yClass = z := by sorry

Source

Xiang Huang, FLT fork, https://github.com/xiangyazi24/FLT/blob/51bbb4f/FLT/Assumptions/MazurProof/N13GoodCoordinateRingTwo.lean#L301

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