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Every automorphism acts on the commutator layer with the eigenvalue one (Theorem 11)

Proved
RInf.induced_eigen_one

by Lucas · Oct 22, 2026 · Mathlib 0df444a (Lean v4.33.1)

eigenvaluesrepresentation-theorysymmetric-groups

Every automorphism of the pure Artin braid group acts on the commutator layer with the eigenvalue one: the induced action equals the value of the symmetric-group representation at some permutation by the factorization of equation (8), and every value of the representation has a nonzero fixed vector - the content of Theorems 8 and 11, where the representation is identified with the irreducible representation of the symmetric group of Young diagram shape with three single cells, and the eigenvalue one is read off through the character analysis of Section 3.

Preamble
import Mathlib
import Definitions.Def_RInf_alg
import Definitions.Def_RInf_braid
Formal statement
namespace RInf

/-!  Milestone 5 (Theorem 11): every automorphism of the pure braid group acts
on the commutator layer with the eigenvalue one. The induced action of α
equals the representation value rep σ for some permutation σ by the
factorization of equation (8), and every value of the representation has a
nonzero fixed vector - the content of Theorems 8 and 11, where the
representation is identified with the irreducible representation of the
symmetric group of Young diagram shape with three single cells, and the
eigenvalue one is read off from the cyclic exponents via Stembridge's
theorem. -/
theorem induced_eigen_one {n : Nat} {P Q : Type*} [Group P] [AddCommGroup Q]
    (W : BraidWorld n P Q) (α : P ≃* P) : HasEigenOne (W.act α) := by sorry

end RInf
Source
K. Dekimpe, D. L. Goncalves, O. Ocampo, The R-infinity property for pure Artin braid groups, Monatshefte fur Mathematik 195 (2021) 15-33, Section 3 (Theorems 8 and 11).
Read-back

What the Lean code literally says, in plain math · glm-5-latest-short

Restate: for every automorphism of the pure braid group of the world, the induced automorphism of the commutator layer has the eigenvalue one: some nonzero element of the layer is fixed. Hypotheses: the braid world is given; no further hypotheses appear.

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