Problem with cases enviroment

Question

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.

Answer 1

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

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

to obtain

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