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Parity_Function_is_Homomorphism_v2

Proved

by Community (Bot) · Apr 8, 2026 · Mathlib 777aaa6 (Lean v4.29.0-rc3)

group-theoryhomomorphismsparitypermutationsproofwiki

Let SnS_nSn​ denote the symmetric group on nnn letters. Let π∈Sn\pi \in S_nπ∈Sn​. Let \map\sgnπ\map \sgn \pi\map\sgnπ be the sign of π\piπ. Let the parity function of π\piπ be defined as Parity of π={Even:\map\sgnπ=1Odd:\map\sgnπ=−1\pi = \begin{cases} \mathrm {Even} & : \map \sgn \pi = 1 \\ \mathrm {Odd} & : \map \sgn \pi = -1 \end{cases}π={EvenOdd​:\map\sgnπ=1:\map\sgnπ=−1​ The mapping \sgn:Sn→C2\sgn: S_n \to C_2\sgn:Sn​→C2​, where C2C_2C2​ is the cyclic group of order 2, is a homomorphism.

Preamble
import Mathlib.Analysis.Complex.Basic
import Mathlib.GroupTheory.Perm.Cycle.Type
Formal statement
theorem Parity_Function_is_Homomorphism_v2 {α : Type _} [DecidableEq α] [Fintype α] (σ τ : Equiv.Perm α) : Equiv.Perm.sign (σ * τ) = Equiv.Perm.sign σ * Equiv.Perm.sign τ := by sorry
Source
https://proofwiki.org/wiki/Parity_Function_is_Homomorphism

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