# Problem with cases enviroment

I am not able to pinpoint where the problem is - it seems that [] are the culprit there. Anyway only one of the following almost identical codes is rendered. (At the moment.)

$$\Gamma(x)=\begin{cases} [0,1]\setminus\mathbb Q; &x<0 \\ [0,1]; & x\ge 0 \end{cases}$$


$$\Gamma(x)=\begin{cases} [0,1]\setminus\mathbb Q; &x<0 \\ [0,1]; & x\ge 0 \end{cases}$$

$$\Gamma(x)=\begin{cases} (0,1)\setminus\mathbb Q; &x<0 \\ (0,1); & x\ge 0 \end{cases}$$


$$\Gamma(x)=\begin{cases} (0,1)\setminus\mathbb Q; &x<0 \\ (0,1); & x\ge 0 \end{cases}$$

$$\Gamma(x)=\begin{cases} [0,1]\setminus\mathbb Q; &x<0 \\ \left[0,1\right]; & x\ge 0 \end{cases}$$


$$\Gamma(x)=\begin{cases} [0,1]\setminus\mathbb Q; &x<0 \\ \left[0,1\right]; & x\ge 0 \end{cases}$$

I've noticed this in an old post of mine. Although I cannot be absolutely sure, I believe it was rendered ok, when I posted it (quite some time ago). There is some discussion in the comments bellow my answer, which means that I've returned to that post a few times, so I would have probably noticed the problem.

A screenshot - just in case this is browser-dependent or this behavior will change.

• It also happens on $\LaTeX$ sometimes; I run into this sort of problem all the time when I have equations involving commutators. It thinks you are giving it an optional parameter. – Arturo Magidin Apr 20 '12 at 14:21

The problem is that MathJax is incorrectly thinking that your \\ is followed by an optional parameter because the next line starts with an open bracket (note that \\[dimen] is used to add space between lines). Normally, optional parameters can have space before the [, and new-lines count as space, but for \\[dimen] it should not be allowed. This is a bug that will be fixed in the next release of MathJax, but for now you can use
$$\Gamma(x)=\begin{cases} [0,1]\setminus\mathbb Q; &x<0 \\ {[0,1]}; & x\ge 0 \end{cases}$$
`
$$\Gamma(x)=\begin{cases} [0,1]\setminus\mathbb Q; &x<0 \\ {[0,1]}; & x\ge 0 \end{cases}$$