# nLab tensor product of commutative monoids

Contents

### Context

#### Algebra

higher algebra

universal algebra

## Theorems

#### Monoidal categories

monoidal categories

With braiding

With duals for objects

With duals for morphisms

With traces

Closed structure

Special sorts of products

Semisimplicity

Morphisms

Internal monoids

Examples

Theorems

In higher category theory

#### Monoid theory

monoid theory in algebra:

# Contents

## Idea

For $A$ and $B$ two commutative monoids, their tensor product $A \otimes B$ is a new commutative monoid such that a monoid homomorphism $A \otimes B \to C$ is equivalently a bilinear map out of $A$ and $B$.

## Definition

###### Definition

Let CMon be the collection of commutative monoids, regarded as a multicategory whose multimorphisms are the multilinear maps $A_1, \cdots, A_n \to B$.

The tensor product $A, B \mapsto A \otimes B$ in this multicategory is the tensor product of commutative monoids.

Equivalently this means explicitly:

###### Definition

For $A, B$ two commutative monoids, their tensor product of commutative monoids is the commutative monoid $A \otimes B$ which is the quotient of the free commutative monoid on the product of their underlying sets $A \times B$ by the relations

• $(a_1,b)+(a_2,b)\sim (a_1+a_2,b)$

• $(a,b_1)+(a,b_2)\sim (a,b_1+b_2)$

• $(0,b)\sim 0$

• $(a,0)\sim 0$

for all $a, a_1, a_2 \in A$ and $b, b_1, b_2 \in B$.

###### Remark

By definition of the free construction and the quotient there is a canonical function of the underlying sets

$p_{A,B} \;\colon\; U(A) \times U(B) \overset{}{\longrightarrow} U(A \otimes B)$

(where $U \colon CMon \to Set$ is the forgetful functor).

On elements this sends $(a,b)$ to the equivalence class that it represents under the above equivalence relations.

The following relates the tensor product to bilinear functions. It is a definition or a proposition dependening on whether one takes the notion of bilinear function to be defined before or after that of tensor product of commutative monoids.

###### Definition/Proposition

A function of underlying sets $f : A \times B \to C$ is a bilinear function precisely if it factors by the morphism of through a monoid homomorphism $\phi : A \otimes B \to C$ out of the tensor product:

$f : A \times B \stackrel{\otimes}{\to} A \otimes B \stackrel{\phi}{\to} C \,.$

## Properties

### Symmetric monoidal category structure

###### Proposition

Equipped with the tensor product $\otimes$ of def. and the exchange map $\sigma_{A, B}: A\otimes B \to B \otimes A$ generated by $\sigma_{A, B}(a, b) = (b, a)$, CMon becomes a symmetric monoidal category.

The unit object in $(CMon, \otimes)$ is the additive monoid of natural numbers $\mathbb{N}$.

###### Proof

To see that $\mathbb{N}$ is the unit object, consider for any commitative momoid $A$ the map

$A \otimes \mathbb{N} \to A$

which sends for $n \in \mathbb{N}$

$(a, n) \mapsto n \cdot a \coloneqq \underbrace{a + a + \cdots + a}_{n\;summands} \,.$

Due to the quotient relation defining the tensor product, the element on the left is also equal to

$(a, n) = (a, \underbrace{1 + 1 \cdots + 1}_{n\; summands}) = \underbrace{ (a,1) + (a,1) + \cdots + (a,1) }_{n\; summands} \,.$

This shows that $A \otimes \mathbb{N} \to A$ is in fact an isomorphism.

Showing that $\sigma_{A, B}$ is natural in $A, B$ is trivial, so $\sigma$ is a braiding. $\sigma^2$ is identity, so it gives CMon a symmetric monoidal structure.

###### Proposition

The tensor product of commutative monoids distributes over the biproduct of commutative monoids

$A \otimes \oplus_{s \in S} B_s \simeq \oplus_{s \in S} ( A \otimes B_s ) \,.$

### Monoids

###### Proposition

A monoid in $(CMon, \otimes)$ is equivalently a rig.

###### Proof

Let $(A, \cdot)$ be a monoid in $(CMon, \otimes)$. The fact that the multiplication

$\cdot : A \otimes A \to A$

is bilinear means by the above that for all $a_1, a_2, b \in A$ we have

$(a_1 + a_2) \cdot b = a_1 \cdot b + a_2 \cdot b$
$b \cdot (a_1 + a_2) = b \cdot a_1 + b \cdot a_2 \,.$
$0 \cdot b = 0 \,.$
$b \cdot 0 = 0 \,.$

This is precisely the distributivity law and absorption law? of the rig.

Last revised on July 18, 2023 at 09:20:30. See the history of this page for a list of all contributions to it.