Matrices not rendered correctly

Question

See e.g. here. I tried both \begin{pmatrix}... and \begin{array}{cc}....

edit Reading the answers, the problem turns out to be the special treatment of backslashes by markdown, which should be turned off (or auto-escaped) inside TeX environments.

Answer 1

$\begin{matrix}a&b&c\\ d&e&f\\ g&h&i\end{matrix}$

\begin{matrix}a&b&c\\\ d&e&f\\\ g&h&i\end{matrix}

Note that I am forced to escape the backslashes. This is a bug.

To do a pmatrix, we would do:

$\begin{pmatrix}a&b&c\\ d&e&f\\ g&h&i\end{pmatrix}$

\begin{pmatrix}a&b&c\\\ d&e&f\\\ g&h&i\end{pmatrix}

On MO, we can surround all tex by backticks to automatically escape all reserved characters like so:

'$\begin{matrix}a&b&c\\ d&e&f\\ g&h&i\end{matrix}$' (replace the apostrophes with backticks)

The array solution proposed by Tom is deprecated in mathematical documents (see the amsmath documentation).

Solution

---

A good solution would be as follows: The administrators should automatically escape all text within dollar signs before it is served to MathJax.

Answer 2

EDIT: This approach has been deprecated. However, according to the amsmath documentation, page 10: "If you need left or right alignment in a column or other special formats you must resort to [the] array [environment]." The accepted solution has been provided above.

---

So, it looks like this works using the array environment. I am pretty sure this site is supporting specific latex packages (ex. amsmath, etc.) but I don't know which ones. At the end of the day, I am confident that we have a robust system at our fingertips - we just have to figure it out!!

So, the code for matrices appears to be something like this:

$A = \left[ \begin{array}{rrr} 1 & -1 & 1 \\ -1 & 1 & -1 \\ 0 & 1 & 2 \end{array} \right]$

$A = \left[ \begin{array}{rrr} 
1  & -1 & 1 \\\
-1 &  1 & -1 \\\
0  &  1 &  2 
\end{array} \right]$

Notice the {rrr} after beginning the array environment. This specifies that we will have three columns, each one right-justified (other options should be well known, l=left-justified, c=centered). Also, there is an extra backslash because this rendering software interprets the first one as an escape character.

Thanks to all of those who have worked very hard to get this site up and running with pretty darn good latex support.

Answer 3

$\\\$ <- Three slashes between dollar signs

This will be escaped* in new (or edited) posts. This is not yet reflected in the preview pane. This is also a per-site setting, this behavior does not exist on sites without client-side LaTeX rendering enabled.

_{*Technically, it just bypasses markdown conversion entirely.}