Previewed post does not match rendered post when MathJax uses *

Question

In revision 2 of Number of permutations in $S_{a + b}$ with $\operatorname{maj}(\pi) = b$ and $\operatorname{maj}(\pi^{-1}) = b$, I edited an equation that had been manually given the label (\ast) to use \tag. As usual when I have to make up a label for an equation that wasn't already given one, I used the existing tag; in this case, \label{*}, with the corresponding \eqref{*}. This worked fine in the generated preview, but, when the post actually rendered, the * inside \eqref was parsed as Markdown rather than as MathJax.

@GeraldEdgar's test suggests that this might not happen on MMO (or maybe we're not exercising the specific buggy conditions here?), but visiting the specific revision https://mathoverflow.net/revisions/384561/2 shows that it does happen on MO.

In case TeX in the main post affects @GeraldEdgar's test, $1 + 2 = 3$. In case you missed it, \begin{equation} \tag{*}\label{*} 1 + 2 = 3. \end{equation} In case you still missed it, refer back to \eqref{}. As @AntonGeraschenko points out, the problem is in referring to \eqref{} twice.

Since neither @AntonGeraschenko nor I am able to reproduce the problematic behaviour I report—that is, where the preview does not match the rendering—I probably made a mistake. I have accepted their answer, but if it would be better to close this question, then I have no objection to doing so.

Answer 1

I'm not able to reproduce the mismatch between preview and render. When I go to edit the question and paste the source of that revision, I get the same (undesirable) rendering in the preview. Are you sure the preview worked before?

Diagnosis

The problem is that you used two *'s outside of math mode. The markdown processing (i.e. converting markdown to bold, italics, headings, etc.) happens first (server side if you're not using the preview), and then MathJax processing happens. Inside dollar signs, markdown explicitly avoids things it would normally process, on the assumption that stuff is meant for MathJax to consume. But outside of math mode (or raw code mode), the * in \eqref{*} looks like fair game. When you use two of them, the markdown processor happily converts them to <em> and </em> and then MathJax has no chance.

Proposed solution 1: Don't use markdown special characters in \label{}. The label you use isn't visible anyway (only the tag is).

Note also that when all the the MathJax on the page is processed together (which doesn't happen in preview), repeated labels will cause "Label multiply defined" errors in MathJax, because MathJax don't know anything about the semantics of questions and answers ... it's just one big web page to it. However, the same tag being repeated is totally fine (albeit potentially confusing to the reader).

Proposed solution 2: Put your \eqref{} in dollar signs so that the markdown processor ignores it.

Demonstrations

Problem: (Note I'm using _ instead of * to avoid global collision with the label in your question)

$$\begin{equation} A = B \tag{*}\label{_} \end{equation}$$
Look at equation \eqref{_}; isn't equation \eqref{_} awesome?

produces the following output:

$$\begin{equation} A = B \tag{*}\label{_} \end{equation}$$ Look at equation \eqref{}; isn't equation \eqref{} awesome?

Solution 1:

$$\begin{equation} A = B \tag{*}\label{ast} \end{equation}$$
Look at equation \eqref{ast}; isn't equation \eqref{ast} awesome?

$$\begin{equation} A = B \tag{*}\label{ast} \end{equation}$$ Look at equation \eqref{ast}; isn't equation \eqref{ast} awesome?

Solution 2: (Note I'm using __ instead of _ to avoid collision with the problem demonstration)

$$\begin{equation} A = B \tag{*}\label{__} \end{equation}$$
Look at equation $\eqref{__}$; isn't equation $\eqref{__}$ awesome?

$$\begin{equation} A = B \tag{*}\label{__} \end{equation}$$ Look at equation $\eqref{__}$; isn't equation $\eqref{__}$ awesome?

Global collision example:

$$\begin{equation} A = B \tag{*}\label{*} \end{equation}$$

$$\begin{equation} A = B \tag{*}\label{*} \end{equation}$$

Answer 2

Trying to reproduce the problem. An equation $$ 4+2=9 \tag{*}\label{1} $$ and then reference to it \eqref{1}. How does it look?

Another equation $$ 4\times2 = 6 \tag{**}\label{2} $$ and a reference to it \eqref{2}. How does it look?

OK when the page is first rendered, or when the page is re-rendered later. But not OK following an edit with no re-render of the whole page. And I expect problems when there is other LaTeX above this on the same page.