lean2/library/theories/finite_group_theory/hom.lean

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/-
Copyright (c) 2015 Haitao Zhang. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Author : Haitao Zhang
-/
import algebra.group data.set .subgroup
namespace group_theory
-- ⁻¹ in eq.ops conflicts with group ⁻¹
-- open eq.ops
notation H1 ▸ H2 := eq.subst H1 H2
open set
open function
open group_theory.ops
open quot
local attribute set [reducible]
section defs
variables {A B : Type}
variable [s1 : group A]
variable [s2 : group B]
include s1
include s2
-- the Prop of being hom
definition homomorphic [reducible] (f : A → B) : Prop := ∀ a b, f (a*b) = (f a)*(f b)
-- type class for inference
structure is_hom_class [class] (f : A → B) : Type :=
(is_hom : homomorphic f)
-- the proof of hom_prop if the class can be inferred
definition is_hom (f : A → B) [h : is_hom_class f] : homomorphic f :=
@is_hom_class.is_hom A B s1 s2 f h
definition ker (f : A → B) [h : is_hom_class f] : set A := {a : A | f a = 1}
definition isomorphic (f : A → B) := injective f ∧ homomorphic f
structure is_iso_class [class] (f : A → B) extends is_hom_class f : Type :=
(inj : injective f)
lemma iso_is_inj (f : A → B) [h : is_iso_class f] : injective f:=
@is_iso_class.inj A B s1 s2 f h
lemma iso_is_iso (f : A → B) [h : is_iso_class f] : isomorphic f:=
and.intro (iso_is_inj f) (is_hom f)
end defs
section
variables {A B : Type}
variable [s1 : group A]
definition id_is_iso [instance] : @is_hom_class A A s1 s1 (@id A) :=
is_hom_class.mk (take a b, rfl)
variable [s2 : group B]
include s1
include s2
variable f : A → B
variable [h : is_hom_class f]
include h
theorem hom_map_one : f 1 = 1 :=
have P : f 1 = (f 1) * (f 1), from
calc f 1 = f (1*1) : mul_one
... = (f 1) * (f 1) : is_hom f,
eq.symm (mul.right_inv (f 1) ▸ (mul_inv_eq_of_eq_mul P))
theorem hom_map_inv (a : A) : f a⁻¹ = (f a)⁻¹ :=
have P : f 1 = 1, from hom_map_one f,
have P1 : f (a⁻¹ * a) = 1, from (eq.symm (mul.left_inv a)) ▸ P,
have P2 : (f a⁻¹) * (f a) = 1, from (is_hom f a⁻¹ a) ▸ P1,
have P3 : (f a⁻¹) * (f a) = (f a)⁻¹ * (f a), from eq.symm (mul.left_inv (f a)) ▸ P2,
mul_right_cancel P3
theorem hom_map_mul_closed (H : set A) : mul_closed_on H → mul_closed_on (f ' H) :=
assume Pclosed, assume b1, assume b2,
assume Pb1 : b1 ∈ f ' H, assume Pb2 : b2 ∈ f ' H,
obtain a1 (Pa1 : a1 ∈ H ∧ f a1 = b1), from Pb1,
obtain a2 (Pa2 : a2 ∈ H ∧ f a2 = b2), from Pb2,
have Pa1a2 : a1 * a2 ∈ H, from Pclosed a1 a2 (and.left Pa1) (and.left Pa2),
have Pb1b2 : f (a1 * a2) = b1 * b2, from calc
f (a1 * a2) = f a1 * f a2 : is_hom f a1 a2
... = b1 * f a2 : {and.right Pa1}
... = b1 * b2 : {and.right Pa2},
mem_image Pa1a2 Pb1b2
lemma ker.has_one : 1 ∈ ker f := hom_map_one f
lemma ker.has_inv : subgroup.has_inv (ker f) :=
take a, assume Pa : f a = 1, calc
f a⁻¹ = (f a)⁻¹ : by rewrite (hom_map_inv f)
... = 1⁻¹ : by rewrite Pa
... = 1 : by rewrite one_inv
lemma ker.mul_closed : mul_closed_on (ker f) :=
take x y, assume (Px : f x = 1) (Py : f y = 1), calc
f (x*y) = (f x) * (f y) : by rewrite [is_hom f]
... = 1 : by rewrite [Px, Py, mul_one]
lemma ker.normal : same_left_right_coset (ker f) :=
take a, funext (assume x, begin
esimp [ker, set_of, glcoset, grcoset],
rewrite [*(is_hom f), mul_eq_one_iff_mul_eq_one (f a⁻¹) (f x)]
end)
definition ker_is_normal_subgroup : is_normal_subgroup (ker f) :=
is_normal_subgroup.mk (ker.has_one f) (ker.mul_closed f) (ker.has_inv f)
(ker.normal f)
-- additional subgroup variable
variable {H : set A}
variable [is_subgH : is_subgroup H]
include is_subgH
theorem hom_map_subgroup : is_subgroup (f ' H) :=
have Pone : 1 ∈ f ' H, from mem_image (@subg_has_one _ _ H _) (hom_map_one f),
have Pclosed : mul_closed_on (f ' H), from hom_map_mul_closed f H subg_mul_closed,
have Pinv : ∀ b, b ∈ f ' H → b⁻¹ ∈ f ' H, from
assume b, assume Pimg,
obtain a (Pa : a ∈ H ∧ f a = b), from Pimg,
have Painv : a⁻¹ ∈ H, from subg_has_inv a (and.left Pa),
have Pfainv : (f a)⁻¹ ∈ f ' H, from mem_image Painv (hom_map_inv f a),
and.right Pa ▸ Pfainv,
is_subgroup.mk Pone Pclosed Pinv
end
section hom_theorem
variables {A B : Type}
variable [s1 : group A]
variable [s2 : group B]
include s1
include s2
variable {f : A → B}
variable [h : is_hom_class f]
include h
definition ker_nsubg [instance] : is_normal_subgroup (ker f) :=
is_normal_subgroup.mk (ker.has_one f) (ker.mul_closed f)
(ker.has_inv f) (ker.normal f)
definition quot_over_ker [instance] : group (coset_of (ker f)) := mk_quotient_group (ker f)
-- under the wrap the tower of concepts collapse to a simple condition
example (a x : A) : (x ∈ a ∘> ker f) = (f (a⁻¹*x) = 1) := rfl
lemma ker_coset_same_val (a b : A): same_lcoset (ker f) a b → f a = f b :=
assume Psame,
have Pin : f (b⁻¹*a) = 1, from subg_same_lcoset_in_lcoset a b Psame,
have P : (f b)⁻¹ * (f a) = 1, from calc
(f b)⁻¹ * (f a) = (f b⁻¹) * (f a) : (hom_map_inv f)
... = f (b⁻¹*a) : by rewrite [is_hom f]
... = 1 : by rewrite Pin,
eq.symm (inv_inv (f b) ▸ inv_eq_of_mul_eq_one P)
definition ker_natural_map : (coset_of (ker f)) → B :=
quot.lift f ker_coset_same_val
example (a : A) : ker_natural_map ⟦a⟧ = f a := rfl
lemma ker_coset_hom (a b : A) :
ker_natural_map (⟦a⟧*⟦b⟧) = (ker_natural_map ⟦a⟧) * (ker_natural_map ⟦b⟧) := calc
ker_natural_map (⟦a⟧*⟦b⟧) = ker_natural_map ⟦a*b⟧ : rfl
... = f (a*b) : rfl
... = (f a) * (f b) : by rewrite [is_hom f]
... = (ker_natural_map ⟦a⟧) * (ker_natural_map ⟦b⟧) : rfl
lemma ker_map_is_hom : homomorphic (ker_natural_map : coset_of (ker f) → B) :=
take aK bK,
quot.ind (λ a, quot.ind (λ b, ker_coset_hom a b) bK) aK
lemma ker_coset_inj (a b : A) : (ker_natural_map ⟦a⟧ = ker_natural_map ⟦b⟧) → ⟦a⟧ = ⟦b⟧ :=
assume Pfeq : f a = f b,
have Painb : a ∈ b ∘> ker f, from calc
f (b⁻¹*a) = (f b⁻¹) * (f a) : by rewrite [is_hom f]
... = (f b)⁻¹ * (f a) : by rewrite (hom_map_inv f)
... = (f a)⁻¹ * (f a) : by rewrite Pfeq
... = 1 : by rewrite (mul.left_inv (f a)),
quot.sound (@subg_in_lcoset_same_lcoset _ _ (ker f) _ a b Painb)
lemma ker_map_is_inj : injective (ker_natural_map : coset_of (ker f) → B) :=
take aK bK,
quot.ind (λ a, quot.ind (λ b, ker_coset_inj a b) bK) aK
-- a special case of the fundamental homomorphism theorem or the first isomorphism theorem
theorem first_isomorphism_theorem : isomorphic (ker_natural_map : coset_of (ker f) → B) :=
and.intro ker_map_is_inj ker_map_is_hom
end hom_theorem
end group_theory