I need to type something like x^{x^x} in LaTeX, but I keep getting the error
! Double superscript.
l.820 ...}(1)_t^{z_{11}}\mathcal{Q}(1)_t^{\prime}^
{y_{11}}\mathcal{Q}(2)_t^{...
How do I get this solved?
It mostly depends on what you want to achieve; if you want that y_{11} is a superscript to the prime, then type \mathcal{Q}(1)_t^{\prime^{y_{11}}} (but I suspect it's not what you want). If you want the prime followed by y_{11}, then either \mathcal{Q}(1)_t^{\prime y_{11}} or, more simply, \mathcal{Q}(1)_t'^{y_{11}}.
Example
\documentclass{article}
\begin{document}
$\mathcal{Q}(1)_t^{\prime^{y_{11}}}$
\medskip
$\mathcal{Q}(1)_t^{\prime y_{11}}$
\medskip
$\mathcal{Q}(1)_t'^{y_{11}}$
\end{document}
I was just stupid. I did the obvious and in this case the obvious doesn't work. You have to group the double exponential not as x^y^x but as x^{y^z}.
Still another possibility: Write
${\mathcal{Q}(1)_t'}^{y_{11}}$
i.e., place \mathcal{Q}(1)_t' inside curly braces. This raises the y_{11} portion a bit relative to the preceding component. Doing this should help create the impression (assuming, of course, that that's what you want) that y_{11} is a superscript particle to the formula, rather than co-equal with the prime symbol.
