# Partial Orders

A **partial order** is a relation that satisfies three requirments:

- $aRa \forall a \in R$
- $aRb$ and $bRa$ implies $a=b$
- $aRb$ and $bRc$ implies $aRc$

A nonempty set that satisfies the requirements of a partially order is called **partially ordered set**.

It might be difficult right now to imagine a set that is partially ordered, but the real numbers under the relation $\leq$ is a partially ordered set. If $aRb$ is an element of this set, like $5 \leq 7$, we write $5 \prec 7.$

**comparable**- two elements of a partially ordered set are said to be comparable if $a \prec b$ or $b \prec a.$
**totally ordered**- a set is said to be totally ordered if all the elements are comparable

You might want to think of a total ordering with the relation,$<.$

**upper bound**- an element $c \in A$ is said to be an upper bound for $B \subset A$, if $b \prec c$ for all $b \in B$
**least upper bound**- $c$ is a least upper bound for $B$ if for all $d$ which are upper bounds of $B$, $c \prec d.$

The mathematical definitions for upper bound and least upper bound correspond to their English translations.

Finally we come to equivalence relations. A relation is said to be an **equivalence relation** if

- If $a \in A$ then $aRa$
- $aRb$ implies $a=b$
- $aRb$ and $bRc$ implies $aRc.$

Those are the three requirements for an equivalence relation. Equivalence relations differ from a partial order in the second requirement. The transitive property remains the same. You might want to think of the equivalence relations as the = relation which makes sense if you think about the requirements for an equivalence relation.