Definition:Ordering on Extended Real Numbers

From ProofWiki
Jump to: navigation, search

Definition

Let $\overline \R$ denote the extended real numbers.

Extend the natural ordering $\le_\R$ on $\R$ to $\overline \R = \R \cup \left\{{+\infty, -\infty}\right\}$ by imposing:

$\forall x \in \overline \R: -\infty \le x$
$\forall x \in \overline \R: x \le +\infty$

That is, considering the relations $\le$ and $\le_\R$ as subsets of $\overline \R \times \overline \R$:

${\le} := {\le_\R} \cup \left\{{ \left({ x, +\infty }\right): x \in \overline \R}\right\} \cup \left\{{ \left({ -\infty, x }\right): x \in \overline \R}\right\}$

The ordering $\le$ is called the (usual) ordering on $\overline \R$.


Also see