Superscript and subscript in align environment

Question

I have the following:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

I want this:

\begin{align*}
\Omega_1 &= {\sigma_{1}}^{2} \left( X' B_1' A_1' A_1 B_1 X \right)^{-1} \\
\Omega_2 &= {\sigma_{1}}^{2} \left( X^*' B_2' A_2' A_2 B_2 X^* \right)^{-1}
\end{align*}

\end{document}

When I try to compile it, I get:

! Double superscript.
<recently read> ^

I cannot understand why. Any help?

Answer 1

You didn't show the error message as formatted by TeX which is

! Double superscript.
<recently read> ^
                 
l.11 \end{align*}
                 
? 

Math errors in align slightly harder to find than in normal math as for technical reasons the whole environment is read first, so all errors are reported at \end{align}

It often helps to debug by removing the alignment (even if the visual form is then incorrect)

\documentclass{article}
\usepackage{amsmath}

\begin{document}

I want this:

\[
\begin{matrix}
\Omega_1 &= {\sigma_{1}}^{2} \left( X' B_1' A_1' A_1 B_1 X \right)^{-1} \\
\Omega_2 &= {\sigma_{1}}^{2} \left( X^*' B_2' A_2' A_2 B_2 X^* \right)^{-1}
\end{matrix}
\]

\end{document}

produces the error message

! Double superscript.
<recently read> ^
                 
l.11 \Omega_2 &= {\sigma_{1}}^{2} \left( X^*'
                                              B_2' A_2' A_2 B_2 X^* \right)^...

? 

which clearly highlights the exact place the error occurs, X^*' is the same as X^*^{\prime} hence a double superscript.

You could use {X^*}' or X^{*\prime} depending on how you want it to look.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\[
{X^*}' + X^{*\prime}
\]

\end{document}