# Affine monoid

In abstract algebra, a branch of mathematics, an affine monoid is a commutative monoid that is finitely generated, and is isomorphic to a submonoid of a free abelian group $\mathbb {Z} ^{d},d\geq 0$. Affine monoids are closely connected to convex polyhedra, and their associated algebras are of much use in the algebraic study of these geometric objects.

## Characterization

* Affine monoids are finitely generated. This means for a monoid $M$, there exists $m_{1},\dots ,m_{n}\in M$ such that
    $M=m_{1}\mathbb {Z_{+}} +\dots +m_{n}\mathbb {Z_{+}}$.
* Affine monoids are cancellative. In other words,
    $x+y=x+z$ implies that $y=z$ for all $x,y,z\in M$, where $+$ denotes the binary operation on the affine monoid $M$.
* Affine monoids are also torsion free. For an affine monoid $M$, $nx=ny$ implies that $x=y$ for $n\in \mathbb {N}$, and $x,y\in M$.
* A subset $N$ of a monoid $M$ that is itself a monoid with respect to the operation on $M$ is a submonoid of $M$.

### Properties and examples

* Every submonoid of $\mathbb {Z}$ is finitely generated. Hence, every submonoid of $\mathbb {Z}$ is affine.
* The submonoid $\{(x,y)\in \mathbb {Z} \times \mathbb {Z} \mid y>0\}\cup \{(0,0)\}$ of $\mathbb {Z} \times \mathbb {Z}$ is not finitely generated, and therefore not affine.
* The intersection of two affine monoids is an affine monoid.

## Affine monoids
### Group of differences

    If $M$ is an affine monoid, it can be embedded into a group. More specifically, there is a unique group $gp(M)$, called the group of differences, in which $M$ can be embedded.

#### Definition

* $gp(M)$ can be viewed as the set of equivalences classes $x-y$, where $x-y=u-v$ if and only if $x+v+z=u+y+z$, for $z\in M$, and
$(x-y)+(u-v)=(x+u)-(y+v)$ defines the addition.
* The rank of an affine monoid $M$ is the rank of a group of $gp(M)$.
* If an affine monoid $M$ is given as a submonoid of $\mathbb {Z} ^{r}$, then $gp(M)\cong \mathbb {Z} M$, where $\mathbb {Z} M$ is the subgroup of $\mathbb {Z} ^{r}$.

#### Universal property

* If $M$ is an affine monoid, then the monoid homomorphism $\iota :M\to gp(M)$ defined by $\iota (x)=x+0$ satisfies the following universal property:
    for any monoid homomorphism $\varphi :M\to G$, where $G$ is a group, there is a unique group homomorphism $\psi :gp(M)\to G$, such that $\varphi =\psi \circ \iota$, and since affine monoids are cancellative, it follows that $\iota$ is an embedding. In other words, every affine monoid can be embedded into a group.

### Normal affine monoids
#### Definition

* If $M$ is a submonoid of an affine monoid $N$, then the submonoid
    ${\hat {M}}_{N}=\{x\in N\mid mx\in M,m\in \mathbb {N} \}$
is the integral closure of $M$ in $N$. If $M={\hat {M_{N}}}$, then $M$ is integrally closed.
* The normalization of an affine monoid $M$ is the integral closure of $M$ in $gp(M)$. If the normalization of $M$, is $M$ itself, then $M$ is a normal affine monoid.
* A monoid $M$ is a normal affine monoid if and only if $\mathbb {R} _{+}M$ is finitely generated and $M=\mathbb {Z} ^{r}\cap \mathbb {R} _{+}M$ .

## Affine monoid rings

    see also: Group ring

### Definition

* Let $M$ be an affine monoid, and $R$ a commutative ring. Then one can form the affine monoid ring $R[M]$. This is an $R$-module with a free basis $M$, so if $f\in R[M]$, then
    $f=\sum _{i=1}^{n}f_{i}x_{i}$, where $f_{i}\in R,x_{i}\in M$, and $n\in \mathbb {N}$.
    In other words, $R[M]$ is the set of finite sums of elements of $M$ with coefficients in $R$.

## Connection to convex geometry

    Affine monoids arise naturally from convex polyhedra, convex cones, and their associated discrete structures.
* Let $C$ be a rational convex cone in $\mathbb {R} ^{n}$, and let $L$ be a lattice in $\mathbb {Q} ^{n}$. Then $C\cap L$ is an affine monoid. (Lemma 2.9, Gordan's lemma)
* If $M$ is a submonoid of $\mathbb {R} ^{n}$, then $\mathbb {R} _{+}M$ is a cone if and only if $M$ is an affine monoid.
* If $M$ is a submonoid of $\mathbb {R} ^{n}$, and $C$ is a cone generated by the elements of $gp(M)$, then $M\cap C$ is an affine monoid.
* Let $P$ in $\mathbb {R} ^{n}$ be a rational polyhedron, $C$ the recession cone of $P$, and $L$ a lattice in $\mathbb {Q} ^{n}$. Then $P\cap L$ is a finitely generated module over the affine monoid $C\cap L$. (Theorem 2.12)