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Symmetric-group relabelling action on ordered configurations

Definition
TarchaBraids_endpoint_permutation_action_v1

by WillR · Sep 21, 2026 · Mathlib 0df444a (Lean v4.33.1)

braid-groupsconfiguration-spacecovering-spacespermutationstarcha

The symmetric group acts continuously on ordered configurations by relabelling strands. A permutation g sends a labelled configuration p to the configuration whose coordinate function is p composed with g inverse.

Definition code
import Mathlib
import Definitions.Def_BraidsLinksMCG_ConfigSpace

namespace TarchaBraids

open BraidsLinksMCG

def orderedConfigPermute {n : ℕ} (g : Equiv.Perm (Fin n)) (p : OrderedConfig n) :
    OrderedConfig n :=
  ⟨p.1 ∘ g.symm, p.2.comp g.symm.injective⟩

instance orderedConfigPermMulAction (n : ℕ) :
    MulAction (Equiv.Perm (Fin n)) (OrderedConfig n) where
  smul g p := orderedConfigPermute g p
  one_smul p := by
    apply Subtype.ext
    rfl
  mul_smul g h p := by
    apply Subtype.ext
    rfl

instance orderedConfigPermContinuous (n : ℕ) :
    ContinuousConstSMul (Equiv.Perm (Fin n)) (OrderedConfig n) where
  continuous_const_smul g := by
    apply Continuous.subtype_mk
    exact continuous_pi fun i =>
      (continuous_apply (g.symm i)).comp continuous_subtype_val

end TarchaBraids
Source
Standard ordered-configuration model of braid groups and the symmetric-group deck action.

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