Defining a group given by generators and relations

Given a subset rels of relations of the free group on a type α, this file constructs the group given by generators x : α and relations r ∈ rels.

Main definitions

• PresentedGroup rels: the quotient group of the free group on a type α by a subset rels of relations of the free group on α.
• of: The canonical map from α to a presented group with generators α.
• toGroup f: the canonical group homomorphism PresentedGroup rels → G, given a function f : α → G from a type α to a group G which satisfies the relations rels.

Tags

generators, relations, group presentations

def PresentedGroup {α : Type u_1} (rels : Set (FreeGroup α)) : Type u_1

Given a set of relations, rels, over a type α, PresentedGroup constructs the group with generators x : α and relations rels as a quotient of FreeGroup α.

Equations

• PresentedGroup rels = (FreeGroup α ⧸ Subgroup.normalClosure rels)

instance PresentedGroup.instGroup {α : Type u_1} (rels : Set (FreeGroup α)) : Group (PresentedGroup rels)

Equations

• PresentedGroup.instGroup rels = QuotientGroup.Quotient.group (Subgroup.normalClosure rels)

def PresentedGroup.of {α : Type u_1} {rels : Set (FreeGroup α)} (x : α) : PresentedGroup rels

of is the canonical map from α to a presented group with generators x : α. The term x is mapped to the equivalence class of the image of x in FreeGroup α.

Equations

• PresentedGroup.of x = ↑(FreeGroup.of x)

@[simp]

theorem PresentedGroup.closure_range_of {α : Type u_1} (rels : Set (FreeGroup α)) : Subgroup.closure (Set.range PresentedGroup.of) = ⊤

The generators of a presented group generate the presented group. That is, the subgroup closure of the set of generators equals ⊤.

theorem PresentedGroup.closure_rels_subset_ker {α : Type u_1} {G : Type u_2} [Group G] {f : α → G} {rels : Set (FreeGroup α)} (h : ∀ r ∈ rels, (FreeGroup.lift f) r = 1) : Subgroup.normalClosure rels ≤ (FreeGroup.lift f).ker

theorem PresentedGroup.to_group_eq_one_of_mem_closure {α : Type u_1} {G : Type u_2} [Group G] {f : α → G} {rels : Set (FreeGroup α)} (h : ∀ r ∈ rels, (FreeGroup.lift f) r = 1) (x : FreeGroup α) : x ∈ Subgroup.normalClosure rels → (FreeGroup.lift f) x = 1

def PresentedGroup.toGroup {α : Type u_1} {G : Type u_2} [Group G] {f : α → G} {rels : Set (FreeGroup α)} (h : ∀ r ∈ rels, (FreeGroup.lift f) r = 1) : PresentedGroup rels →* G

The extension of a map f : α → G that satisfies the given relations to a group homomorphism from PresentedGroup rels → G.

Equations

• PresentedGroup.toGroup h = QuotientGroup.lift (Subgroup.normalClosure rels) (FreeGroup.lift f) ⋯

@[simp]

theorem PresentedGroup.toGroup.of {α : Type u_1} {G : Type u_2} [Group G] {f : α → G} {rels : Set (FreeGroup α)} (h : ∀ r ∈ rels, (FreeGroup.lift f) r = 1) {x : α} : (PresentedGroup.toGroup h) (PresentedGroup.of x) = f x

theorem PresentedGroup.toGroup.unique {α : Type u_1} {G : Type u_2} [Group G] {f : α → G} {rels : Set (FreeGroup α)} (h : ∀ r ∈ rels, (FreeGroup.lift f) r = 1) (g : PresentedGroup rels →* G) (hg : ∀ (x : α), g (PresentedGroup.of x) = f x) {x : PresentedGroup rels} : g x = (PresentedGroup.toGroup h) x

theorem PresentedGroup.ext_iff {α : Type u_1} {G : Type u_2} [Group G] {rels : Set (FreeGroup α)} {φ : PresentedGroup rels →* G} {ψ : PresentedGroup rels →* G} : φ = ψ ↔ ∀ (x : α), φ (PresentedGroup.of x) = ψ (PresentedGroup.of x)

theorem PresentedGroup.ext {α : Type u_1} {G : Type u_2} [Group G] {rels : Set (FreeGroup α)} {φ : PresentedGroup rels →* G} {ψ : PresentedGroup rels →* G} (hx : ∀ (x : α), φ (PresentedGroup.of x) = ψ (PresentedGroup.of x)) : φ = ψ

def PresentedGroup.equivPresentedGroup {α : Type u_1} {β : Type u_3} (rels : Set (FreeGroup α)) (e : α ≃ β) : PresentedGroup rels ≃* PresentedGroup (⇑(FreeGroup.freeGroupCongr e) '' rels)

Presented groups of isomorphic types are isomorphic.

Equations

• PresentedGroup.equivPresentedGroup rels e = QuotientGroup.congr (Subgroup.normalClosure rels) (Subgroup.normalClosure (⇑(FreeGroup.freeGroupCongr e) '' rels)) (FreeGroup.freeGroupCongr e) ⋯

theorem PresentedGroup.equivPresentedGroup_apply_of {α : Type u_1} {β : Type u_3} (x : α) (rels : Set (FreeGroup α)) (e : α ≃ β) : (PresentedGroup.equivPresentedGroup rels e) (PresentedGroup.of x) = PresentedGroup.of (e x)

theorem PresentedGroup.equivPresentedGroup_symm_apply_of {α : Type u_1} {β : Type u_3} (x : β) (rels : Set (FreeGroup α)) (e : α ≃ β) : (PresentedGroup.equivPresentedGroup rels e).symm (PresentedGroup.of x) = PresentedGroup.of (e.symm x)

instance PresentedGroup.instInhabited {α : Type u_1} (rels : Set (FreeGroup α)) : Inhabited (PresentedGroup rels)

Equations

• PresentedGroup.instInhabited rels = { default := 1 }