# The MathJax newline \\ is broken in tag wikis

In an attempt to edit the tag wiki of I noticed the following strange behaviour.

The conjecture is often stated as a definition by cases and even serves as an example for that notation in the mother of all MathJax demos. I tried to apply that to the tag wiki, by modifying its code to that:

$$a_{i+1} =\begin{cases} \frac{a_i}{2} & \text{if a_i is even}\\[2ex] 3 a_i + 1 & \text{if a_i is odd} \end{cases}$$


Which renders just fine here:

$$a_{i+1} =\begin{cases} \frac{a_i}{2} & \text{if a_i is even}\\[2ex] 3 a_i + 1 & \text{if a_i is odd} \end{cases}$$

However, in the tag wiki, not so much:

As can be seen by the output, there seems to be a problem with the newline which is not interpreted as such. I added the test case

$$a\\b$$


which also renders properly outside tag wikis:

$$a\\b$$

The fact that the above behaviour is only present in the tag wiki makes me suspect that it's a bug. Of course, pebkac is always an option and this might be expected behaviour/by design/my fault. Either way, can somebody please explain this?

• that's peculiar ... Could it be related to the fact that in standard LaTeX the \[ is used to start a new math block? At least it seems to choke on that ... – Vogel612 Oct 8 '16 at 22:27
• @Vogel612 I had tried it without the additional spacing information (that is the [2ex] for those that don't know), but that didn't work either. Also, the a\\b test case hints at the problem being somewhere else. However, your comment let me start a bit more experimenting and I could reproduce, the problem outside the tag wiki, will post that as an answer for now. – I'll add comments tomorrow Oct 8 '16 at 22:33
• flagged: It's been almost a month now and there is no answer from the people in charge whatsoever. No confirmation if this is a bug or not, if somebody is working on it, if it is of too low priority to be fixed now, etc. I hoped providing the results of my own investigations would help solve the problem quickly. This was apparently not the case. – I'll add comments tomorrow Nov 6 '16 at 19:41

I managed to reproduce the problem outside the tag wiki.

If one takes the regularly working code:

$$a_{i+1} =\begin{cases} \frac{a_i}{2} & \text{if a_i is even}\\[2ex] 3 a_i + 1 & \text{if a_i is odd} \end{cases}$$


$$a_{i+1} =\begin{cases} \frac{a_i}{2} & \text{if a_i is even}\\[2ex] 3 a_i + 1 & \text{if a_i is odd} \end{cases}$$

and removes one backslash:

$$a_{i+1} =\begin{cases} \frac{a_i}{2} & \text{if a_i is even}\[2ex] 3 a_i + 1 & \text{if a_i is odd} \end{cases}$$


$$a_{i+1} =\begin{cases} \frac{a_i}{2} & \text{if a_i is even}\[2ex] 3 a_i + 1 & \text{if a_i is odd} \end{cases}$$

the result looks the same as with two backslashes in the tag wiki. I suspect that some process somehow swallows backslashes, before the input is parsed by the mathjax engine.

Please be careful brave stackexchange developers, the backslash swallowing monster is loose!

On a more serious note, merely removing a backslash from the second test case

$$a\b$$


$$a\b$$

creates a visible error (stuff showing up in red) on the output.

This is different from the output seen in the screenshot in the question, where the a b test case renders without error, but also without the newline.

To reproduce that, one has to insert an additional space:

$$a\ b$$


$$a\ b$$

et voilà, the newline goes missing, without any visible error.

I suspect that the aforementioned process not only swallows backslashes but produces additional spaces as a result.

Beware! It's the attack of the backslash-swallowing-space-pooping mathjax monster. Fly, you fools!

On another more serious note, could somebody with more insights on the code please tame the beast?

You were right about it swallowing backslashes. You need to use a real real backslash:

$$a\\\b$$