# Groups and family

open import Types.relations
open import Types.equality renaming (refl to ≡-refl)
open import Types.functions
open import Types.product

open import Agda.Primitive using (Level; _⊔_; lsuc; lzero)

module Algebra.groups {a ℓ} {A : Set a} (_==_ : Rel A ℓ) where

open import Types.operations (_==_)

Group-like objects form another family of objects probed in abstract algebra. They have a similar pattern of building like ordered objects but are more complex. This family contains of a set of type A and a binary operation ⋅ defined on that set which satisfy a bunch of properties. As there are a large number of properties to choose from, one can end up with a large number of families, however here we describe only a few that generally matter.

Object ↓ Laws → Totality Associativity Identity Invertibility Commutativity
Magma
Semigroupoid
Small Category
Quasigroup
Loop
Semigroup
Inverse Semigroup
Groupoid
Monoid
Group
Abelian group

Note that we implement only the packaged version of laws here, the actual object types we define in the next section Groups and family 2, this is precisely because we cannot have two high level modules per agda file.

As we see above, semigroupoid is a generalization of the semigroup, group is a stricter form of a monoid or all groups are also monoids etc. Semigroupoids, Small Categories and Groupoids form what is called as partial abstract algebra such that they don’t require the totality property which thus allows their operations to be partial functions.

## Magma

A magma is a set of objects with a closed binary operation defined on them. It is one of the simplest objects in abstract algebra.

A magma is a structure containing:

• A set $$𝔽$$
• A binary operation: ∙

where:

1. ∙ is closed, i.e. ∀ x y ∈ 𝔽, (x ∙ y) ∈ 𝔽 or ∙ : 𝔽 × 𝔽 → 𝔽
  record IsMagmaMinimal (∙ : ★ A) : Set (a ⊔ ℓ) where

However, we define a more constrained magma, where:

1. The set 𝔽 has an underlying equivalence relation == (can simply use setoids instead)
2. ∙ is congruent over the underlying equality ==
3. ∙ is closed, i.e. ∀ x y ∈ 𝔽, (x ∙ y) ∈ 𝔽 or ∙ : 𝔽 × 𝔽 → 𝔽
  record IsMagma (∙ : ★ A) : Set (a ⊔ ℓ) where
field
∙-cong        : Congruent₂ ∙
isEquivalence : IsEquivalence _==_

open IsEquivalence isEquivalence public

-- utils
∙-congˡ : LeftCongruent ∙
∙-congˡ y==z = ∙-cong y==z rfl

∙-congʳ : RightCongruent ∙
∙-congʳ y==z = ∙-cong rfl y==z

## Semigroup

A semigroup is a structure where the operation is associative.

A magma is a structure containing:

• A set $$𝔽$$
• A binary operation: ∙

where:

1. ∙ is closed, i.e. ∀ x y ∈ 𝔽, (x ∙ y) ∈ 𝔽 or ∙ : 𝔽 × 𝔽 → 𝔽
2. ∙ is associative, i.e. x ∙ (y ∙ z) == (x ∙ y) ∙ z
  record IsSemigroup (∙ : ★ A) : Set (a ⊔ ℓ) where
field
isMagma : IsMagma ∙
assoc   : Associative ∙

open IsMagma isMagma public

## Monoid

A monoid is a semigroup, with a special element called the identity element. Monoids and semigroups are perhaps of most significance to programmers as these are widely used to model various types and operations on them. For example, JSON objects and a merge operation on them form a monoid. Strings and string concatenation form monoids too.

Here are a few examples of monoids:

Object Operation Identity
int subtraction 0
int multiplication 1
float subtraction 0.0
float multiplication 1.0
JSON merge two JSONs {}
JSON create / delete / modify a key-value pair {}
JSON λ : JSON<A, B>, JSON<A, B> → JSON<A, B> {}
string concatenation ''
List / Array concatenation []

Monoidal operations (∙ : A × A → A) take two elements and “reduce” or “combine” them into one. In other words they can be used to model types that can be aggregated, as one could take successive pairs of objects from a monoid and combine them with the monoidal operation ∙ : A × A → A. This is reflected in the fact that only “certain types” may be “reduced” using “certain type” of functions. The “reduce” here refers to a standard map-reduce operation.

A monoid is a structure containing:

• A set $$𝔽$$
• A binary operation: ∙

where:

1. ∙ is closed, i.e. ∀ x y ∈ 𝔽, (x ∙ y) ∈ 𝔽 or ∙ : 𝔽 × 𝔽 → 𝔽
2. ∙ is associative, i.e. x ∙ (y ∙ z) == (x ∙ y) ∙ z
3. ∙ has an identity, i.e. ∃ i ∈ A, i ∙ i = i
  record IsMonoid (∙ : ★ A) (x : A) : Set (a ⊔ ℓ) where
field
isSemigroup : IsSemigroup ∙
identity    : Identity x ∙

open IsSemigroup isSemigroup public

-- utils
identityˡ : LeftIdentity x ∙
identityˡ = fst identity

identityʳ : RightIdentity x ∙
identityʳ = snd identity

## Commutative Monoid

A commutative monoid is a monoid with its operation required to be commutative.

A commutative monoid is a structure containing:

• A set $$𝔽$$
• A binary operation: ∙

where:

1. ∙ is closed, i.e. ∀ x y ∈ 𝔽, (x ∙ y) ∈ 𝔽 or ∙ : 𝔽 × 𝔽 → 𝔽
2. ∙ is associative, i.e. x ∙ (y ∙ z) == (x ∙ y) ∙ z
3. ∙ has an identity, i.e. ∃ i ∈ A, i ∙ i = i
4. ∙ is commutative, i.e. x ∙ y = y ∙ x
  record IsCommutativeMonoid (∙ : ★ A) (x : A) : Set (a ⊔ ℓ) where
field
isMonoid : IsMonoid ∙ x
comm        : Commutative ∙

open IsMonoid isMonoid public

## Group

A group is a monoid with the additional requirement of the binary operation to have an inverse operation for every pair of elements of the group. A group is another widely-used structure.

Field Used to model
Physics Symmetry in Noether’s theorem
Physics Gauge theories (quantum electrodynamics, quantum field theory)
Physics Gauge formulation of general relativity
Physics M-brane theory (and other string theories)
Chemistry Classification of crystal structure
Chemistry Symmetries of molecules
Cryptography ECDSA (Elliptic Curve Digital Signature Algorithm) signatures

A group is a structure containing:

• A set $$𝔽$$
• A binary operation: ∙

where:

1. ∙ is closed, i.e. ∀ x y ∈ 𝔽, (x ∙ y) ∈ 𝔽 or ∙ : 𝔽 × 𝔽 → 𝔽
2. ∙ is associative, i.e. x ∙ (y ∙ z) == (x ∙ y) ∙ z
3. ∙ has an identity, i.e. ∃ i ∈ A, i ∙ i = i
4. every object x has an inverse x⁻¹, such that ((x ⁻¹) ∙ x) == i
  record IsGroup (_∙_ : ★ A) (x : A) (_⁻¹ : ♠ A) : Set (a ⊔ ℓ) where
field
isMonoid  : IsMonoid _∙_ x
inverse   : Inverse x _⁻¹ _∙_
⁻¹-cong   : Congruent₁ _⁻¹

open IsMonoid isMonoid public

infixl 7 _-_
_-_ : ★ A
x - y = x ∙ (y ⁻¹)

inverseˡ : LeftInverse x _⁻¹ _∙_
inverseˡ = fst inverse

inverseʳ : RightInverse x _⁻¹ _∙_
inverseʳ = snd inverse

open import Types.equational2
open withCongruence _==_ _∙_ _⁻¹ rfl trans sym ∙-cong x public

-- uniqueness of the inverses
uniqueˡ-⁻¹ : ∀ α β → (α ∙ β) == x → α == (β ⁻¹)
uniqueˡ-⁻¹ = assoc+id+invʳ⇒invˡ-unique assoc identity inverseʳ

uniqueʳ-⁻¹ : ∀ α β → (α ∙ β) == x → β == (α ⁻¹)
uniqueʳ-⁻¹ = assoc+id+invˡ⇒invʳ-unique assoc identity inverseˡ

## Abelian Group

An Abelian group, named after Niels Henrik Abel, is a group and requires its operation to also be commutative.

An abelian group is a structure containing:

• A set $$𝔽$$
• A binary operation: ∙

where:

1. ∙ is closed, i.e. ∀ x y ∈ 𝔽, (x ∙ y) ∈ 𝔽 or ∙ : 𝔽 × 𝔽 → 𝔽
2. ∙ is associative, i.e. x ∙ (y ∙ z) == (x ∙ y) ∙ z
3. ∙ has an identity, i.e. ∃ i ∈ A, i ∙ i = i
4. ∙ is commutative, i.e. x ∙ y = y ∙ x
5. every object x has an inverse x⁻¹, such that ((x ⁻¹) ∙ x) == i
  record IsAbelianGroup (∙ : ★ A) (x : A) (⁻¹ : ♠ A) : Set (a ⊔ ℓ) where
field
isGroup : IsGroup ∙ x ⁻¹
comm    : Commutative ∙

open IsGroup isGroup public

isCommutativeMonoid : IsCommutativeMonoid ∙ x
isCommutativeMonoid = record
{ isMonoid = isMonoid
; comm = comm
}

Groups and family 2