English
A permutation lies in the closure of the swaps if and only if its fixed-by complement is finite in the translated formulation.
Русский
Перестановка лежит в замыкании транспозиций тогда и только тогда, когда её complemento фиксирования конечна в соответствующей формулировке.
LaTeX
$$theorem mem_closure_isSwap' ...$$
Lean4
/-- If a subgroup is generated by transpositions, then a permutation `f` lies in the subgroup if
and only if `f` has finite support and `f x` always lies in the same orbit as `x`. -/
theorem mem_closure_isSwap {S : Set (Perm α)} (hS : ∀ f ∈ S, f.IsSwap) {f : Perm α} :
f ∈ closure S ↔ (fixedBy α f)ᶜ.Finite ∧ ∀ x, f x ∈ orbit (closure S) x :=
by
refine ⟨fun hf ↦ ⟨?_, fun x ↦ mem_orbit_iff.mpr ⟨⟨f, hf⟩, rfl⟩⟩, ?_⟩
· exact finite_compl_fixedBy_closure_iff.mpr (fun f hf ↦ (hS f hf).finite_compl_fixedBy) _ hf
rintro ⟨fin, hf⟩
set supp := (fixedBy α f)ᶜ with supp_eq
suffices h : (fixedBy α f)ᶜ ⊆ supp → f ∈ closure S from h supp_eq.symm.subset
clear_value supp; clear supp_eq; revert f
apply fin.induction_on ..
· rintro f - emp; convert (closure S).one_mem; ext; by_contra h; exact emp h
rintro a s - - ih f hf supp_subset
refine (mul_mem_cancel_left ((swap_mem_closure_isSwap hS).2 (hf a))).1 (ih (fun b ↦ ?_) fun b hb ↦ ?_)
· rw [Perm.mul_apply, swap_apply_def]; split_ifs with h1 h2
· rw [← orbit_eq_iff.mpr (hf b), h1, orbit_eq_iff.mpr (hf a)]; apply mem_orbit_self
· rw [← orbit_eq_iff.mpr (hf b), h2]; apply hf
· exact hf b
· contrapose! hb
simp_rw [notMem_compl_iff, mem_fixedBy, Perm.smul_def, Perm.mul_apply, swap_apply_def, apply_eq_iff_eq]
by_cases hb' : f b = b
· rw [hb']; split_ifs with h <;> simp only [h]
simp [show b = a by simpa [hb] using supp_subset hb']
