# I can't put a Greek letter into \tag{…}

David Madore said this was a good place to report MathJax bugs, and I think I have encountered one. Here is a formula tagged with the label “$FLT$”:

$$a^n + b^n = x^n \tag{FLT}$$

To get this I used \tag{FLT}.

Now I would like to label the following formula with “$F\Phi$”, analogously using \tag{F\Phi}, but it doesn't work:

$$a^n + b^n = x^n \tag{F\Phi}$$

Is this is a bug or a feature? If it's a feature, why?

• I meant to post this on the meta site. I have flagged it for moderator attention. – MJD Jan 14 '14 at 3:56

Apparently, the expression inside the "tag" is interpreted as a text, not a math formula. Try this:

$$a^n + b^n = x^n \tag{F\Phi}$$

which yields

$$a^n + b^n = x^n \tag{F\Phi}$$

$$x^n+y^n=z^n \tag{F\Phi}$$ Use dollar signs around the F\Phi, since \tag switches to text mode. If you want $F$ upright, there are various fixes.

Why? It would not be attractive if default were math mode, since FLT would become $FLT$.

• Thanks very much. – MJD Jan 14 '14 at 4:16
• You are welcome. At one point I helped a bit with a book on LaTeX, and learned many things that I have mostly forgotten. – André Nicolas Jan 14 '14 at 4:19
• Put the dollar signs only around the \Phi. (F$\Phi$) – Geoff Pointer Jan 14 '14 at 5:15

Is this is a bug or a feature? If it's a feature, why?

It's a feature. Note that that's how $\rm\LaTeX$'s \tag{} macro works (its argument is placed in text mode), so MathJax treats it the same way.