## Monoids

Abbreviation: Mon

### Definition

A monoid is a structure $\mathbf{M}=\langle M,\cdot ,e\rangle$, where $\cdot$ is an infix binary operation, called the monoid product, and $e$ is a constant (nullary operation), called the identity element , such that

$\cdot$ is associative: $(x\cdot y)\cdot z=x\cdot (y\cdot z)$

$e$ is an identity for $\cdot$: $e\cdot x=x$, $x\cdot e=x$.

##### Morphisms

Let $\mathbf{M}$ and $\mathbf{N}$ be monoids. A morphism from $\mathbf{M}$ to $\mathbf{N}$ is a function $h:Marrow N$ that is a homomorphism:

$h(x\cdot y)=h(x)\cdot h(y)$, $h(e)=e$

### Examples

Example 1: $\langle X^{X},\circ ,id_{X}\rangle$, the collection of functions on a sets $X$, with composition, and identity map.

Example 1: $\langle M(V)_{n},\cdot ,I_{n}\rangle$, the collection of $n\times n$ matrices over a vector space $V$, with matrix multiplication and identity matrix.

Example 1: $\langle \Sigma ^{\ast },\cdot ,\lambda \rangle$, the collection of strings over a set $\Sigma$, with concatenation and the empty string. This is the free monoid generated by $\Sigma$.

### Properties

Classtype Variety decidable in polynomial time undecidable undecidable no unbounded no no no no no no no no no

### Finite members

$\begin{array}{lr} f(1)= &1\\ f(2)= &2\\ f(3)= &7\\ f(4)= &35\\ f(5)= &228\\ f(6)= &2237\\ f(7)= &31559\\ \end{array}$