Question about set notation: what does $]a,b[$ mean?

$]a,b[$ is somtimes used to denote the open interval $(a,b)$ i.e. $$]a,b[ = \{x \in \mathbb{R}: x > a \,\,\,\& \,\,\, x <b\}$$

$\;\;]\,a,b\,[\;\;$ is used by some to denote the open interval $(a,b)$.

Put differently, $$\,]\,a,b\,[\,\, = \{x \in \mathbb{R}:\; a\,<x\,<b\},$$ which reads, "the set of all real numbers greater than $a$ but less than $b$".

This notation helps to distinguish the open interval $(a, b)$ from the ordered pair $(a, b)$.

The expression $c\in ]a,b[$ means "$c$ belongs to the open interval from $a$ to $b$". Another (more common) way to denote the same thing is $c \in (a,b)$.

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