Central elements of monoids

Content created by Fredrik Bakke, Egbert Rijke and Maša Žaucer.

Created on 2023-03-18.
Last modified on 2023-03-21.

module group-theory.central-elements-monoids where
open import foundation.identity-types
open import foundation.propositions
open import foundation.universe-levels

open import group-theory.central-elements-semigroups
open import group-theory.monoids


An element x of a monoid M is said to be central if xy = yx for every y : M.


module _
  {l : Level} (M : Monoid l)

  is-central-element-monoid-Prop : type-Monoid M  Prop l
  is-central-element-monoid-Prop =
    is-central-element-semigroup-Prop (semigroup-Monoid M)

  is-central-element-Monoid : type-Monoid M  UU l
  is-central-element-Monoid =
    is-central-element-Semigroup (semigroup-Monoid M)

  is-prop-is-central-element-Monoid :
    (x : type-Monoid M)  is-prop (is-central-element-Monoid x)
  is-prop-is-central-element-Monoid =
    is-prop-is-central-element-Semigroup (semigroup-Monoid M)


The unit element is central

module _
  {l : Level} (M : Monoid l)

  is-central-element-unit-Monoid : is-central-element-Monoid M (unit-Monoid M)
  is-central-element-unit-Monoid y =
    left-unit-law-mul-Monoid M y  inv (right-unit-law-mul-Monoid M y)

The product of two central elements is central

module _
  {l : Level} (M : Monoid l)

  is-central-element-mul-Monoid :
    (x y : type-Monoid M) 
    is-central-element-Monoid M x  is-central-element-Monoid M y 
    is-central-element-Monoid M (mul-Monoid M x y)
  is-central-element-mul-Monoid =
    is-central-element-mul-Semigroup (semigroup-Monoid M)

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