# What does the interval $]-\frac{\pi}{2},\frac{\pi}{2}[$ mean? [duplicate]

It could be a misprint or typo in what I'm reading, but I just want to make sure.

I'm reading Loring Tu's "An Introduction to Manifolds", and at the end of the first chapter, one of the problem states:

$$\textbf{1.3 A diffeomorphism of an open interval with}\,\,\mathbb{R}$$

Let $$U\subset\mathbb{R}^n$$ and $$V\subset\mathbb{R}^n$$. A $$C^\infty$$ map $$F: U\rightarrow V$$ is called a diffeomorphism if it is bijective and has a $$C^\infty$$ inverse $$F^{-1}:V\rightarrow U$$.

(a) Show that the function $$f: \,\,\,]-\pi/2, \pi/2[\rightarrow \mathbb{R}, f(x)=\tan(x)$$ is a diffeomorphism.

To be clear, I'm not asking for help with the problem, just the notation he uses. Elsewhere in the text, it appears that every time he mentions an open interval, he uses this $$]a,b[$$ notation, but for closed intervals he uses the traditional $$[a,b]$$.

Is this notation commonly used, or likely some sort of misprint with the pdf I'm reading from?

• At some places in the world it is the standard notation for open intervals. – quid May 31 at 14:19
• Could you point me to more info about it? This sort of thing seemed difficult to search for when I tried – Calvin Godfrey May 31 at 14:19
• There is really nothing more to it that and inverted bracket is used instead of a parenthesis. I am pretty sure the question is a duplicate which is why I did not answer it. – quid May 31 at 14:20
• You're welcome to close it or delete it, then! – Calvin Godfrey May 31 at 14:21