Subscript notation render error

[amibug]

When I try to use the grouped subscript notation ("_{}"), Mathjax does not render it. For example:

$P'=\frac { U'_{ cd }  ^{ 2 } } { R } $

[dpvc]

It is not the subscript that is the problem, it is the superscript together with the prime on U' (the "double exponent" refers to this). In TeX, a prime like in U' is a superscript, and you can't use two superscripts on the same base (U), so that leads to this warning. If you put this expression into actual TeX or LaTeX, you will get a similar warning.

One solution is to put braces around U' as in $P'=\frac { {U'}_{ cd } ^{ 2 } } { R } $, though the result may be hard to read visually. It might be better to use $ P'=\frac { (U')_{ cd } ^{ 2 } } { R } $ to visually separate the prime from the superscript.