Definition:Real Interval/Unbounded Closed

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

\(\displaystyle \hointr a \to\) \(:=\) \(\displaystyle \set {x \in \R: a \le x}\)
\(\displaystyle \hointl \gets a\) \(:=\) \(\displaystyle \set {x \in \R: x \le a}\)

Also denoted as

The notation using $\infty$ is usual:

\(\displaystyle \hointr a \infty\) \(:=\) \(\displaystyle \set {x \in \R: a \le x}\)
\(\displaystyle \hointl {-\infty} a\) \(:=\) \(\displaystyle \set {x \in \R: x \le a}\)

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


Example $1$

Let $I$ be the unbounded closed real interval defined as:

$I := \hointl \gets 3$

Then $2 \in I$.

Also see