What does $]a,b[$ mean?

Question

I recently encountered the above notation for sets, and I've never encountered it before. What does it refer to?

Answer 1

This is the notation for the open interval $(a,b)$.

Those two notations denote the same thing.

In case you do not know what an interval is, it is the set $\{x:a<x<b\}$ given an order relation $<$.

Answer 2

That just means the open interval $(a,b)$. The brackets facing to the other side should indicate that the borders $a$ and $b$ do not belong to the set.

Answer 3

$]a,b[$ is the set of numbers between $a$ and $b$ with $a$ and $b$ excluded. In symbols like Hasan Saad said $\{x:a\lt x\lt b\}$.

The opposite $[a,b]$ is the set of number between $a$ and $b$ with $a$ and $b$ included. In symbols $\{x:a\le x\le b\}$.