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I recently saw this notation being used in the statement of a theorem of De Lellis and Szekelyhidi: The first sentence of the theorem says:

Let $\Omega$ be an open ball in $\mathbb{R}^n$, $T>0$, and $\overline{e}$ a uniformly continuous function $\Omega\times ]O, T[\rightarrow]O, + \infty[$

I have never seen this notation before $]O,T[$, what does it mean when the brackets are outward facing? Any suggested readings would be appreciated. I don't even know what to look up to find this I tried "Outward facing brackets mathematics" but couldn't find anything. Is this notation just really uncommon?

Ltoll
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2 Answers2

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Reversed brackets indicate open intevals as $]0,1[$ or semi-open intervals as $[0,1[$, the notation are respectively equivalent to $(0,1)$ and $[0,1)$.

user
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Reverse brackets indicate open intervals (or if only used at one end of an interval, a semi-open interval).

In many countries it's the only notation used for that, and it's typically less confusing, I'll just refer to the exchange in the comments of another question from earlier today: Why is the set $\mathcal B=\{\,\mathopen]a,b\mathclose[\mid a,b\in\mathbb Q\,\}$ countable?

Henrik supports the community
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    “Less confusing” is subjective. It may well be so if you’re used to it. I’m not, so in my mental parser those outward-pointing brackets bind to the arrow instead. – amd Aug 25 '18 at 21:43