# Definition:Ray (Order Theory)

## Definition

Let $\struct {S, \preccurlyeq}$ be a totally ordered set.

Let $\prec$ be the reflexive reduction of $\preccurlyeq$.

Let $a \in S$ be any point in $S$.

### Open Ray

The following sets are called open rays or open half-lines:

$\left\{{x \in S: a \prec x}\right\}$ (the strict upper closure of $a$), denoted $a^\succ$
$\left\{{x \in S: x \prec a}\right\}$ (the strict lower closure of $a$), denoted $a^\prec$.

### Closed Ray

The following sets are called closed rays or closed half-lines:

$\left\{{x \in S: a \preccurlyeq x}\right\}$ (the upper closure of $a$), denoted $a^\succcurlyeq$
$\left\{{x \in S: x \preccurlyeq a}\right\}$ (the lower closure of $a$), denoted $a^\preccurlyeq$.

### Upward-Pointing Ray

An upward-pointing ray is a ray which is bounded below:

an open ray $a^\succ:= \left\{{x \in S: a \prec x}\right\}$
a closed ray $a^\succcurlyeq: \left\{{x \in S: a \preccurlyeq x}\right\}$

### Downward-Pointing Ray

A downward-pointing ray is a ray which is bounded above:

an open ray $a^\prec := \left\{{x \in S: x \prec a}\right\}$
a closed ray $a^\preccurlyeq : \left\{{x \in S: x \preccurlyeq a}\right\}$

## Also known as

A ray (either open or closed is also sometimes referred to as a half-line (either open or closed).

The Wirth interval notation is another notation used widely on $\mathsf{Pr} \infty \mathsf{fWiki}$:

$\openint a \to$ for $a^\succ$
$\openint \gets a$ for $a^\prec$
$\hointr a \to$ for $a^\succcurlyeq$
$\hointr \gets a$ for $a^\preccurlyeq$

can also be used.

## Also see

• Results about rays in the context of order theory can be found here.