Homotopy Type Theory
monoid > history (Rev #6)
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Definition
A monoid or A 3 A_3 -algebra in sets consists of
A type A A ,
A basepoint e : A e:A
A binary operation μ : A → A → A \mu : A \to A \to A
A left unitorλ : ∏ ( a : A ) μ ( e , a ) = a \lambda:\prod_{(a:A)} \mu(e,a)=a
A right unitorρ : ∏ ( a : A ) μ ( a , e ) = a \rho:\prod_{(a:A)} \mu(a,e)=a
An asssociatorα : ∏ ( a : A ) ∏ ( b : A ) ∏ ( c : A ) μ ( μ ( a , b ) , c ) = μ ( a , μ ( b , c ) ) \alpha:\prod_{(a:A)} \prod_{(b:A)} \prod_{(c:A)} \mu(\mu(a, b),c)=\mu(a,\mu(b,c))
A 0-truncatorτ 0 : ∏ ( a : A ) ∏ ( b : A ) ∏ ( c : a = b ) ∏ ( d : a = b ) ∑ ( x : c = d ) ∏ ( y : c = d ) x = y \tau_0: \prod_{(a:A)} \prod_{(b:A)} \prod_{(c:a=b)} \prod_{(d:a=b)} \sum_{(x:c=d)} \prod_{(y:c=d)} x=y
Homomorphisms of monoids
A homomorphism of monoids between two monoids A A and B B consists of
A function ϕ : A → B \phi:A \to B such that
The basepoint is preservedϕ ( e A ) = e B \phi(e_A) = e_B
The binary operation is preserved∏ ( a : A ) ∏ ( b : A ) ϕ ( μ A ( a , b ) ) = μ B ( ϕ ( a ) , ϕ ( b ) ) \prod_{(a:A)} \prod_{(b:A)} \phi(\mu_A(a, b)) = \mu_B(\phi(a),\phi(b))
For any monoid A A , the 0-truncator means that the identity types between any two terms of A A are proposition s, and thus the dependent product types
∏ ( a : A ) μ ( e , a ) = a \prod_{(a:A)} \mu(e,a)=a ∏ ( a : A ) μ ( a , e ) = a \prod_{(a:A)} \mu(a,e)=a ∏ ( a : A ) ∏ ( b : A ) ∏ ( c : A ) μ ( μ ( a , b ) , c ) = μ ( a , μ ( b , c ) ) \prod_{(a:A)} \prod_{(b:A)} \prod_{(c:A)} \mu(\mu(a, b),c)=\mu(a,\mu(b,c))
are propositions, and thus contractible because they are inhabited by definition of a monoid.
As a result, given monoids A A and B B , for any function
ϕ λ : ( ∏ ( a : A ) μ ( e A , a ) = a ) → ( ∏ ( b : B ) μ ( e B , b ) = b ) \phi_\lambda:\left(\prod_{(a:A)} \mu(e_A,a)=a\right) \to \left(\prod_{(b:B)} \mu(e_B,b)=b\right)
the left unitor is preserved:
ϕ λ ( λ A ) = λ B \phi_\lambda(\lambda_A) = \lambda_B
because
∏ ( b : B ) μ ( e B , b ) = b \prod_{(b:B)} \mu(e_B,b)=b
is contractible. Likewise, for any function
ϕ ρ : ( ∏ ( a : A ) μ ( a , e A ) = a ) → ( ∏ ( b : B ) μ ( b , e B ) = b ) \phi_\rho:\left(\prod_{(a:A)} \mu(a, e_A)=a\right) \to \left(\prod_{(b:B)} \mu(b, e_B)=b\right)
the right unitor is preserved:
ϕ ρ ( ρ A ) = ρ B \phi_\rho(\rho_A) = \rho_B
because
∏ ( b : B ) μ ( b , e B ) = b \prod_{(b:B)} \mu(b,e_B)=b
is contractible, and for any function
ϕ α : ( ∏ ( a 1 : A ) ∏ ( a 2 : A ) ∏ ( a 3 : A ) μ ( μ ( a 1 , a 2 ) , a 3 ) = μ ( a 1 , μ ( a 2 , a 3 ) ) ) → ( ∏ ( b 1 : B ) ∏ ( b 2 : B ) ∏ ( b 3 : B ) μ ( μ ( b 1 , b 2 ) , b 3 ) = μ ( b 1 , μ ( b 2 , b 3 ) ) ) \phi_\alpha:\left(\prod_{(a_1:A)} \prod_{(a_2:A)} \prod_{(a_3:A)} \mu(\mu(a_1, a_2),a_3)=\mu(a_1,\mu(a_2,a_3))\right) \to \left(\prod_{(b_1:B)} \prod_{(b_2:B)} \prod_{(b_3:B)} \mu(\mu(b_1, b_2),b_3)=\mu(b_1,\mu(b_2,b_3))\right)
the associator is preserved:
ϕ α ( α A ) = α B \phi_\alpha(\alpha_A) = \alpha_B
because
∏ ( b 1 : B ) ∏ ( b 2 : B ) ∏ ( b 3 : B ) μ ( μ ( b 1 , b 2 ) , b 3 ) = μ ( b 1 , μ ( b 2 , b 3 ) ) \prod_{(b_1:B)} \prod_{(b_2:B)} \prod_{(b_3:B)} \mu(\mu(b_1, b_2),b_3)=\mu(b_1,\mu(b_2,b_3))
is contractible. This means that a homomorphism of monoids is also a homomorphism of A 3 A_3 -spaces. Finally, the 0-truncator is always preserved in a function between two sets.
Examples
The integers are a monoid.
Given a set A A , the type of endofunctions A → A A \to A has the structure of an monoid, with basepoint id A id_A , operation function composition.
See also
References
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