The escape sequence should be
$ is what people who already know (La)TeX use. Even more importantly,
$ is what people who already know how to write math on Stack Exchange use — it's what Mathematics SE uses, and it's what all the MathJax documentation on Stack Exchange (mostly on Math meta) uses. Even the upstream MathJax documentation uses
$ in its examples (even though the MathJax preprocessor doesn't have
$ active by default — it has
\(…\) instead). Everybody who typed math during the private beta in the hope that MathJax would be enabled spontaneously used
\$ reduces the discoverability of MathJax. If I suspect that MathJax is enabled but am not sure, I'll type
$a$ — see that it doesn't come out as math, so no, there's no MathJax. When I typed this question I wasn't even sure whether I should type
This is especially bad for comments which have no preview. I just wrote a comment using math, and I spontaneously typed
$ — because that's how it's done everywhere else (except on Electrical Engineering, because it was introduced when the site had been existing for months — and even there I'm unconvinced that this was the right decision, and the inconsistency is causing technical problems too).
It's to be expected that we'll have migrations between Puzzling and Math at some point. Ok, that only concerns a small number of questions, but it would be a gratuitous incompatibility. Still, it's the human dissonance that I'm mostly concerned about, far more than the technical dissonance.
And what's the point of using
\$? To avoid a relatively rare case where two dollar amounts are used in the same paragraph. People already run into trouble when they use an unprotected
* and feel safe, and then use a second one and get italics all of a sudden.
$ would be no worse than that. It's very visible in the preview, too. Using
$ as an active character is no worse than using
* as an active character.
\$ as an active sequence makes the markup language more irregular: everywhere else a backslash quotes the next character. The fact that it's
\$ for inline math but
$$ and not
\$$ for display math has caused some confusion.
\$ is a gratuitous complication, we should avoid it.