Add \[ \] and \( \) as maths delimiters

[jfbercher]

As mentioned in nbconvert/#477 and earlier in notebook#759, \[ \] and \( \) are not recognized as maths delimiters, and the equations not correctly displayed.
This is not intuitive for LaTeX users (as it is now the recommended way to delimit maths) and causes problems when converting to eg html since the contents of those maths blocks is considered as markdown.
The PR attempt to solve that by updating mathjaxutils.js

• the regex MATHSPLIT is updated to account for these new delimiters
• new start/end delimiters are matched

[takluyver]

I'm inclined to say sorry, but no to further additions to Markdown syntax, even if they're now the recommended way to delimit maths in Latex. The more special exceptions we add to Markdown parsing, the messier it gets.

[rgbkrk]

Thank you for taking the time to make a pull request.

The primary reason we can't support \[ is that it is valid Markdown/CommonMark for an escaped bracket.

As written by @jgm:

...the problem with that syntax is that \[ and \( already have clearly defined meanings: they are escaped [ and ( characters, and it is very important that there be a way to escape these characters in certain circumstances.

We all want to support mathematics typesetting. For jupyter maintainers and operators of notebook server deployments, we can't support extensions to the syntax that are unambiguous. This does mean having to wait it out for others to solve within CommonMark or getting involved there.

[rgbkrk]

All that being said, it's worth looking at

\[ \] and \( \) are not recognized as maths delimiters, and the equations not correctly displayed.

as a usability issue where the notebook could detect likely LaTeX and provide a notification/suggestion to use $$ and $. I'm not volunteering to write that though. 😉