Reading comments from OP, it seems that OP wants the ability to number or label individual lines. In an array
or matrix
the rows aren't numbered/labeled, but in the align
environment, getting the arrows and subsequent terms centered on the \underbrace
is tricky. (Note that this leads to some unnatural spacing in the second line; it is my understanding that this is desired.)
Here is a solution that sets two lengths: \lena
and \lenb
, for the first and second underbraces respectively, then centers subsequent portions using \makebox
and the prescribed widths.
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{calc} % needed for \widthof command
\newlength{\lena}
\newlength{\lenb}
\begin{document}
\setlength{\lena}{\widthof{$\underbrace{(A_0)^2}$}}
\setlength{\lenb}{\widthof{$\underbrace{\sigma_0^2}$}}
\begin{align}
&\mathrel{\phantom{=}}{\underbrace{(A_0)^2}}-\underbrace{\sigma_0^2}<K\label{thisone}\\
&\mathrel{\phantom{=}}\makebox[\lena]{$\downarrow$}\mathbin{\phantom{-}}\makebox[\lenb]{$\downarrow$}\nonumber\\
&=\makebox[\lena]{$U_1$}-\makebox[\lenb]{$U_2$}\label{thatone}
\end{align}
Lines \ref{thisone} and \ref{thatone} can be referenced.
\end{document}

As mentioned in another answer, info on longer downarrows is here.