So in Jekyll and Markdown, I can\'t type $ or its HTML equivalent code, and not \\$ nor $\\$$. How should I type the dollar sign?
To prevent MathJax from using the dollar sign as a delimiter, you can put it in a span, like so:
$
Alternatively, you can enable processEscapes and then you can use the backslash escape \$ for the dollar sign.
processEscapes
\$