I'm a relative beginner with Lyx. When I type in this code I'm hoping for T' to be followed by superscripts and subscripts. Instead I get a "double superscript" error when I try to view the pdf
\[
T'_{\alpha_{1}\alpha_{2}\ldots\alpha_{n}}^{\mu_{1}\mu_{2}\ldots\mu_{m}}
\]
Any idea how I can fix this?
You can insert \prime symbol as part of the superscript ^{...}, rather than having "two superscripts":
\documentclass{article}
\begin{document}
\[
T_{\alpha_{1}\alpha_{2}\ldots\alpha_{n}}^{\prime\mu_{1}\mu_{2}\ldots\mu_{m}}
\]
\end{document}
' in math mode expands to ^\prime, so your code is equivalent to T^{\prime}_{...}^{...}, which explains the error you get. You can either swap sub- and superscripts to be T'^{...}_{...} (' is defined in a way that merges it with an adjacent superscript), or use \prime inside the superscript as suggested in Werner's answer, so both
T'^{\mu_{1}\mu_{2}\ldots\mu_{m}}_{\alpha_{1}\alpha_{2}\ldots\alpha_{n}}
and
T_{\alpha_{1}\alpha_{2}\ldots\alpha_{n}}^{\prime\mu_{1}\mu_{2}\ldots\mu_{m}}
are valid.
In mathmode, ' acts like ^\prime. So you have two superscripts, which isn't allowed. One way around this is to group the T' like so:
\[
{T'}_{\alpha_{1}\alpha_{2}\ldots\alpha_{n}}^{\mu_{1}\mu_{2}\ldots\mu_{m}}
\]
But this doesn't look good. I'd suggest maybe replacing T' with some other variable name…
As Khaled has pointed out, this solution breaks the horizontal positioning of subscripts:
The top one is my grouping solution, the bottom is doing it The Right Way. Given the special service the prime superscript performs (marking out T' as an entity distinct from T, I'm not sure that this solution isn't in fact better.
Either way, I stick to my suggestion that you should consider using a different variable name.
{T'} breaks horizontal placement of subscripts as it puts it in a box of its own with all its side effects, so it is generally a bad idea.
– خالد حسني
Oct 21 '11 at 00:20