' acts like
^\prime. So you have two superscripts, which isn't allowed. One way around this is to group the
T' like so:
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.