open import 1Lab.Prelude

open import Algebra.Magma

module Algebra.Semigroup where

Semigroups🔗

record is-semigroup {A : Type ℓ} (_⋆_ : A → A → A) : Type ℓ where

A semigroup is an associative magma, that is, a set equipped with a choice of associative binary operation ⋆.

  field
    has-is-magma : is-magma _⋆_
    associative : {x y z : A} → x ⋆ (y ⋆ z) ≡ (x ⋆ y) ⋆ z

  open is-magma has-is-magma public

open is-semigroup public

To see why the set truncation is really necessary, it helps to explicitly describe the expected structure of a “∞-semigroup” in terms of the language of higher categories:

An ∞-groupoid A, equipped with
A map _⋆_ : A → A → A, such that
⋆ is associative: there exists an invertible 2-morphism α : A ⋆ (B ⋆ C) ≡ (A ⋆ B) ⋆ C (called the associator), satisfying
The pentagon identity, i.e. there is a path π (called, no joke, the “pentagonator”) witnessing commutativity of the diagram below, where all the faces are α:

The pentagonator satisfies its own coherence law, which looks like the Stasheff polytope $K_5$ , and so on, “all the way up to infinity”.

By explicitly asking that A be truncated at the level of sets, we have that the associator automatically satisfies the pentagon identity - because all parallel paths in a set are equal. Furthermore, by the upwards closure of h-levels, any further coherence condition you could dream up and write down for these morphisms is automatically satisfied.

As a consequence of this truncation, we get that being a semigroup operator is a property of the operator:

is-semigroup-is-prop : {_⋆_ : A → A → A} → is-prop (is-semigroup _⋆_)
is-semigroup-is-prop x y i .has-is-magma =
  is-magma-is-prop (x .has-is-magma) (y .has-is-magma) i
is-semigroup-is-prop {_⋆_ = _⋆_} x y i .associative {a} {b} {c} =
  x .has-is-set (a ⋆ (b ⋆ c)) ((a ⋆ b) ⋆ c) (x .associative) (y .associative) i

instance
  H-Level-is-semigroup : ∀ {_*_ : A → A → A} {n} → H-Level (is-semigroup _*_) (suc n)
  H-Level-is-semigroup = prop-instance is-semigroup-is-prop

A semigroup structure on a type packages up the binary operation and the axiom in a way equivalent to a structure.

Semigroup-on : Type ℓ → Type ℓ
Semigroup-on X = Σ (is-semigroup {A = X})

Semigroup-on is a univalent structure, because it is equivalent to a structure expressed as a structure description. This is only the case because is-semigroup is a proposition, i.e. Semigroup-on can be expressed as a “structure part” (the binary operation) and an “axiom part” (the associativity)!

module _ where
  private
    sg-desc : Str-desc ℓ ℓ (λ X → (X → X → X)) ℓ
    sg-desc .Str-desc.descriptor = s∙ s→ (s∙ s→ s∙)
    sg-desc .Str-desc.axioms X = is-semigroup
    sg-desc .Str-desc.axioms-prop X s = is-semigroup-is-prop

  Semigroup-str : Structure ℓ (Semigroup-on {ℓ = ℓ})
  Semigroup-str = Desc→Str sg-desc

  Semigroup-str-is-univalent : is-univalent (Semigroup-str {ℓ = ℓ})
  Semigroup-str-is-univalent = Desc→is-univalent sg-desc

One can check that the notion of semigroup homomorphism generated by Semigroup-str corresponds exactly to the expected definition, and does not have any superfluous information:

module _
  {A : Type} {_⋆_ : A → A → A} {as : is-semigroup _⋆_}
  {B : Type} {_*_ : B → B → B} {bs : is-semigroup _*_}
  {f : A ≃ B}
  where

  _ : Semigroup-str .is-hom (A , _⋆_ , as) (B , _*_ , bs) f
    ≡ ( (x y : A) → f .fst (x ⋆ y) ≡ (f .fst x) * (f .fst y))
  _ = refl

The “min” semigroup🔗

An example of a naturally-occuring semigroup are the natural numbers under taking minimums.

open import Data.Nat

Nat-min : is-semigroup min
Nat-min .has-is-magma .has-is-set = Nat-is-set
Nat-min .associative = min-assoc _ _ _

What is meant by “naturally occuring” is that this semigroup can not be made into a monoid: There is no natural number unit such that, for all y, min unit y ≡ y and min y unit ≡ y.

private
  min-no-id : (unit : Nat) → ((y : Nat) → min unit y ≡ y) → ⊥
  min-no-id x id =
    let
      sucx≤x : suc x ≤ x
      sucx≤x = subst (λ e → e ≤ x) (id (suc x)) (min-≤l x (suc x))
    in ¬sucx≤x x sucx≤x