Quadratic Twist Var Change base Change map
WeierstrassCurve.quadraticTwistVarChange_baseChange_map
Plain-language statement
The M-level form of the twist's defining cocycle: any σ ∈ Aut(M/K) not fixing L pointwise (i.e. with χ(σ) = -1) conjugates the base change to M of quadraticTwistVarChange by the automorphism [-1] of E. This is the base change of quadraticTwistVarChange_map.
Exact Lean statement
lemma quadraticTwistVarChange_baseChange_map {σ : M ≃ₐ[K] M}
(hσ : ¬ ∀ x : L, σ (algebraMap L M x) = algebraMap L M x) :
((E.quadraticTwistVarChange L).baseChange M).map σ.toAlgHom.toRingHom
= (E.quadraticTwistVarChange L).baseChange M * (E.baseChange M).negVariableChangeFormal artifact
Lean source
lemma quadraticTwistVarChange_baseChange_map {σ : M ≃ₐ[K] M} (hσ : ¬ ∀ x : L, σ (algebraMap L M x) = algebraMap L M x) : ((E.quadraticTwistVarChange L).baseChange M).map σ.toAlgHom.toRingHom = (E.quadraticTwistVarChange L).baseChange M * (E.baseChange M).negVariableChange := by obtain ⟨σ₀, hσ₀⟩ := exists_algEquiv_ne_one K L have hres : σ.restrictNormal L = σ₀ := (algEquiv_eq_one_or_eq K L hσ₀ _).resolve_left (fun h ↦ hσ ((forall_apply_algebraMap_iff_restrictNormal_eq_one K L M σ).mpr h)) have hcomp : σ.toAlgHom.toRingHom.comp (algebraMap L M) = (algebraMap L M).comp σ₀.toAlgHom.toRingHom := by ext l have h := (AlgEquiv.restrictNormal_commutes σ L l).symm rw [hres] at h simpa using h have e1 : ((E.quadraticTwistVarChange L).baseChange M).map σ.toAlgHom.toRingHom = (E.quadraticTwistVarChange L).map (σ.toAlgHom.toRingHom.comp (algebraMap L M)) := (E.quadraticTwistVarChange L).map_map (algebraMap L M) σ.toAlgHom.toRingHom have e2 : ((E.quadraticTwistVarChange L) * (E.baseChange L).negVariableChange).map (algebraMap L M) = (E.quadraticTwistVarChange L).baseChange M * (E.baseChange L).negVariableChange.map (algebraMap L M) := map_mul (VariableChange.mapHom (algebraMap L M)) _ _ rw [e1, hcomp, ← VariableChange.map_map, E.quadraticTwistVarChange_map L hσ₀, e2, negVariableChange_map, baseChange_map_algebraMap]- Project
- Fermat's Last Theorem
- License
- Apache-2.0
- Commit
- 8dd808888295
- Source
- FLT/KnownIn1980s/EllipticCurves/QuadraticTwists/QuadraticTwists.lean:620-643
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