bicyclic semigroup

The bicyclic semigroup $\mathcal{C}({p},{q})$ is the monoid generated by $\{p,q\}$ with the single relation $pq=1$ .

The elements of $\mathcal{C}({p},{q})$ are all words of the form $q^{n}p^{m}$ for $m,n\geq 0$ (with the understanding $p^{0}=q^{0}=1$ ). These words are multiplied as follows:

q^{n}p^{m}q^{k}p^{l}=\begin{cases}q^{n+k-m}p^{l}&\text{if }m\leq k,\\ q^{n}p^{l+m-k}&\text{if }m\geq k.\end{cases}

It is apparent that $\mathcal{C}({p},{q})$ is simple, for if $q^{n}p^{m}$ is an element of $\mathcal{C}({p},{q})$ , then $1=p^{n}(q^{n}p^{m})q^{m}$ and so $S^{1}q^{n}p^{m}S^{1}=S$ .

It is also easy to see that $\mathcal{C}({p},{q})$ is an inverse semigroup: the element $q^{n}p^{m}$ has inverse $q^{m}p^{n}$ .

It is useful to picture some further properties of $\mathcal{C}({p},{q})$ by arranging the elements in a table:

\begin{matrix}1&p&p^{2}&p^{3}&p^{4}&\dots\\ q&qp&qp^{2}&qp^{3}&qp^{4}&\dots\\ q^{2}&q^{2}p&q^{2}p^{2}&q^{2}p^{3}&q^{2}p^{4}&\dots\\ q^{3}&q^{3}p&q^{3}p^{2}&q^{3}p^{3}&q^{3}p^{4}&\dots\\ q^{4}&q^{4}p&q^{4}p^{2}&q^{4}p^{3}&q^{4}p^{4}&\dots\\ \vdots&\vdots&\vdots&\vdots&\vdots&\ddots\end{matrix}

Then the elements below any horizontal line drawn through this table form a right ideal and the elements to the right of any vertical line form a left ideal. Further, the elements on the diagonal are all idempotents and their standard ordering is

1>qp>q^{2}p^{2}>q^{3}p^{3}>\cdots.

Title	bicyclic semigroup
Canonical name	BicyclicSemigroup
Date of creation	2013-03-22 13:09:57
Last modified on	2013-03-22 13:09:57
Owner	mclase (549)
Last modified by	mclase (549)
Numerical id	8
Author	mclase (549)
Entry type	Definition
Classification	msc 20M99
Synonym	bicyclic monoid
Defines	bicyclic