FoxDifferential.exact_foxBoundaryMap_zcGroupLike_sub_one_of_topologicallyGenerates
If a finite family topologically generates \(H\), the corresponding completed finite Fox boundary is exact at \(\mathbb{Z}_C\llbracket H\rrbracket\).
Source context 11 commands
Surrounding namespaces, section variables, and local commands at this declaration.
namespace FoxDifferential
noncomputable section
open scoped Topology
open ProCGroups.Generation
universe u v
section TailExactness
variable (C : ProCGroups.FiniteGroupClass.{u})
variable [ProCGroups.FiniteGroupClass.ContainsTrivialQuotients C]
variable {X : Type v} [Fintype X] [DecidableEq X]
variable {H : Type u} [Group H] [TopologicalSpace H] [IsTopologicalGroup H]
omit [DecidableEq X] inLean source · full declaration
theorem exact_foxBoundaryMap_zcGroupLike_sub_one_of_topologicallyGenerates
(hForm : ProCGroups.FiniteGroupClass.Formation C)
(φ : X → H) (hφ : TopologicallyGenerates (G := H) (Set.range φ)) :
Function.Exact
(foxBoundaryMap (fun x : X => zcGroupLike C H (φ x) - 1) :
(X → ZCCompletedGroupAlgebra C H) → ZCCompletedGroupAlgebra C H)
(zcCompletedGroupAlgebraAugmentation C H :
ZCCompletedGroupAlgebra C H → ZCCoeff C) := by
classical
let L : (X → ZCCompletedGroupAlgebra C H) →ₗ[ZCCompletedGroupAlgebra C H]
ZCCompletedGroupAlgebra C H :=
foxBoundaryMap (fun x : X => zcGroupLike C H (φ x) - 1)
have hclosedRange :
IsClosed ((LinearMap.range L : Submodule (ZCCompletedGroupAlgebra C H)
(ZCCompletedGroupAlgebra C H)) : Set (ZCCompletedGroupAlgebra C H)) := by
change IsClosed (Set.range L)
have hrange :
Set.range L = (fun v : X → ZCCompletedGroupAlgebra C H => L v) '' Set.univ := by
ext y
constructor
· rintro ⟨v, rfl⟩
exact ⟨v, trivial, rfl⟩
· rintro ⟨v, _hv, rfl⟩
exact ⟨v, rfl⟩
rw [hrange]
simpa [L] using
(isCompact_univ.image (continuous_foxBoundaryMap
(fun x : X => zcGroupLike C H (φ x) - 1))).isClosed
let K : Subgroup H :=
{ carrier := {h | zcGroupLike C H h - 1 ∈ LinearMap.range L}
one_mem' := by
change zcCompletedGroupAlgebraBoundary C (MonoidHom.id H) (1 : H) ∈
LinearMap.range L
simp only [MonoidHom.id_apply, zcCompletedGroupAlgebraBoundary_eq_zero_of_mem_ker, zero_mem]
mul_mem' := by
intro a b ha hb
change zcCompletedGroupAlgebraBoundary C (MonoidHom.id H) (a * b) ∈
LinearMap.range L
rw [ScalarCrossedHom.map_mul
(zcCompletedGroupAlgebraBoundary C (MonoidHom.id H))]
exact (LinearMap.range L).add_mem ha ((LinearMap.range L).smul_mem _ hb)
inv_mem' := by
intro a ha
change zcCompletedGroupAlgebraBoundary C (MonoidHom.id H) a⁻¹ ∈
LinearMap.range L
rw [ScalarCrossedHom.map_inv
(zcCompletedGroupAlgebraBoundary C (MonoidHom.id H))]
exact (LinearMap.range L).neg_mem ((LinearMap.range L).smul_mem _ ha) }
have hKclosed : IsClosed ((K : Subgroup H) : Set H) := by
change IsClosed {h : H | zcGroupLike C H h - 1 ∈
(LinearMap.range L : Submodule (ZCCompletedGroupAlgebra C H)
(ZCCompletedGroupAlgebra C H))}
exact hclosedRange.preimage
((continuous_zcGroupLike (C := C) (G := H)).sub continuous_const)
have hsub : Subgroup.closure (Set.range φ) ≤ K := by
rw [Subgroup.closure_le]
rintro h ⟨x, rfl⟩
change zcGroupLike C H (φ x) - 1 ∈ LinearMap.range L
exact ⟨Pi.single x (1 : ZCCompletedGroupAlgebra C H), by
simp only [foxBoundaryMap_single, L]⟩
have htop : (⊤ : Subgroup H) ≤ K := by
have hcl : (Subgroup.closure (Set.range φ)).topologicalClosure ≤ K :=
Subgroup.topologicalClosure_minimal _ hsub hKclosed
rw [TopologicallyGenerates] at hφ
simpa [hφ] using hcl
have hstandard_le_range :
(zcCompletedGroupAlgebraStandardAugmentationIdeal C H :
Submodule (ZCCompletedGroupAlgebra C H) (ZCCompletedGroupAlgebra C H)) ≤
LinearMap.range L := by
rw [zcCompletedGroupAlgebraStandardAugmentationIdeal_eq_span]
refine Submodule.span_le.2 ?_
rintro _ ⟨h, rfl⟩
simpa [K, zcCompletedGroupAlgebraBoundary] using
htop (show h ∈ (⊤ : Subgroup H) from by simp only [Subgroup.mem_top])
have hrange_le_standard :
LinearMap.range L ≤
(zcCompletedGroupAlgebraStandardAugmentationIdeal C H :
Submodule (ZCCompletedGroupAlgebra C H) (ZCCompletedGroupAlgebra C H)) := by
rintro y ⟨v, rfl⟩
rw [zcCompletedGroupAlgebraStandardAugmentationIdeal_eq_span]
change L v ∈ Submodule.span (ZCCompletedGroupAlgebra C H)
(Set.range fun h : H => zcGroupLike C H h - 1)
rw [show L v =
∑ x : X, v x * (zcGroupLike C H (φ x) - 1) from rfl]
exact Submodule.sum_mem _ fun x _ =>
Submodule.smul_mem _ (v x)
(Submodule.subset_span ⟨φ x, rfl⟩)
have haugmentation_le_range :
zcCompletedGroupAlgebraAugmentationIdeal C H ≤
(LinearMap.range L : Submodule (ZCCompletedGroupAlgebra C H)
(ZCCompletedGroupAlgebra C H)) := by
intro z hz
have hzClosure :
z ∈ closure
((zcCompletedGroupAlgebraStandardAugmentationIdeal C H :
Ideal (ZCCompletedGroupAlgebra C H)) : Set (ZCCompletedGroupAlgebra C H)) := by
rw [closure_zcCompletedGroupAlgebraStandardAugmentationIdeal_eq_augmentationIdeal
(C := C) (H := H) hForm]
exact hz
exact closure_minimal
(by intro y hy; exact hstandard_le_range hy) hclosedRange hzClosure
intro z
constructor
· intro hz
exact haugmentation_le_range
((mem_zcCompletedGroupAlgebraAugmentationIdeal_iff
(C := C) (H := H) (x := z)).2 hz)
· rintro ⟨x, rfl⟩
have hstd :
L x ∈ zcCompletedGroupAlgebraStandardAugmentationIdeal C H :=
hrange_le_standard ⟨x, rfl⟩
have haug :
L x ∈ zcCompletedGroupAlgebraAugmentationIdeal C H :=
zcCompletedGroupAlgebraStandardAugmentationIdeal_le_augmentationIdeal C H hstd
exact (mem_zcCompletedGroupAlgebraAugmentationIdeal_iff
(C := C) (H := H) (x := L x)).1 haug