Definition:Real Interval/Unbounded Open

From ProofWiki
Jump to navigation Jump to search


There are two unbounded open intervals involving a real number $a \in \R$, defined as:

\(\ds \openint a \to\) \(:=\) \(\ds \set {x \in \R: a < x}\)
\(\ds \openint \gets a\) \(:=\) \(\ds \set {x \in \R: x < a}\)


An arbitrary (real) interval is frequently denoted $\mathbb I$.

Sources which use the $\textbf {boldface}$ font for the number sets $\N, \Z, \Q, \R, \C$ tend also to use $\mathbf I$ for this entity.

Some sources merely use the ordinary $\textit {italic}$ font $I$.

Some sources prefer to use $J$.

Wirth Interval Notation

The notation used on this site to denote a real interval is a fairly recent innovation, and was introduced by Niklaus Emil Wirth:

\(\ds \openint a b\) \(:=\) \(\ds \set {x \in \R: a < x < b}\) Open Real Interval
\(\ds \hointr a b\) \(:=\) \(\ds \set {x \in \R: a \le x < b}\) Half-Open (to the right) Real Interval
\(\ds \hointl a b\) \(:=\) \(\ds \set {x \in \R: a < x \le b}\) Half-Open (to the left) Real Interval
\(\ds \closedint a b\) \(:=\) \(\ds \set {x \in \R: a \le x \le b}\) Closed Real Interval

The term Wirth interval notation has consequently been coined by $\mathsf{Pr} \infty \mathsf{fWiki}$.

Also denoted as

The notation using $\infty$ is usual:

\(\ds \openint a \infty\) \(:=\) \(\ds \set {x \in \R: a < x}\)
\(\ds \openint {-\infty} a\) \(:=\) \(\ds \set {x \in \R: x < a}\)

On $\mathsf{Pr} \infty \mathsf{fWiki}$ the $\gets \cdots \to$ notation is preferred.

Also known as

Some sources refer to the unbounded open real intervals as open infinite (real) intervals.

Some sources use the term semi-infinite intervals.

Also see