This answer works when you use the fleqn
package. The example below defines two LaTeX macros \mymidline
and \mylastline
. Both macros essentially expand to their first argument within align*
environments. The \mymidline
macro centers it and the \mylastline
macro right-aligns it. Thereby, the width of the stuff in the second argument is subtracted from the available "display width". For align*
environments that should just be the (longest) left-hand side of the equation. The example below shows how you can do that most efficiently with a macro \LHS
.
\documentclass{article}
\usepackage[DIV15]{typearea}
\usepackage{amsmath,amsfonts}
\usepackage{fleqn}
\usepackage{ulem}
\makeatletter
\newdimen\@tzadima
\newdimen\@tzadimb
\newbox\@tzaboxa
\def\mylinemeasures#1#2{%
\@tzadima\displaywidth%
\advance\@tzadima-\tagwidth@%
\advance\@tzadima-\alignsep@%
\setbox\@tzaboxa\hbox{$\displaystyle#1$}%
\@tzadimb\wd\@tzaboxa%
\advance\@tzadima-\@tzadimb%
\setbox\@tzaboxa\hbox{$\displaystyle#2$}%
\@tzadimb\wd\@tzaboxa%
\advance\@tzadima-\@tzadimb%
}
\def\mymidline#1#2{%
\mylinemeasures{#1}{#2}%
\divide\@tzadima2%
\hbox to \@tzadima{}#1\notag
}
\def\mylastline#1#2{%
\mylinemeasures{#1}{#2}%
\hbox to \@tzadima{}#1%
}
\makeatother
\begin{document}
\begin{align*}
\gdef\LHS{(L\cdot R)^{(i)}[i+1:n,i+1:n]}\LHS
&= \underbrace{L^{(i-1)}[i+1:n,1:i-1]\cdot R^{(i-1)}[1:i-1,i+1:n]}_{\text{untouched}}+\\
&\mymidline{+ L^{(i)}[i+1:n,i] \underbrace{R^{(i)}[i,i+1:n]}_{\text{untouched pivot row}}+}\LHS\\
&\mylastline{+ \underbrace{L^{(i)}[i+1:n,i+1:n]}_{=1_{n-i-1}}\cdot R^{(i)}[i+1:n,i+1:n]}\LHS\\
&= L^{(i-1)}[i+1:n,1:i-1]\cdot R^{(i-1)}[1:i-1,i+1:n]
+\\
&\mymidline{+ \uwave{L^{(i)}[i+1:n,i]\cdot R^{(i-1)}[i,i+1:n]}+}\LHS\\
&\mylastline{+R^{(i-1)}[i+1:n,i+1:n]\uwave{\strut- L^{(i)}[i+1:n,i]\cdot R^{(i-1)}[i,i+1:n]}}\LHS\\
&=L^{(i-1)}[i+1:n,1:i-1]\cdot R^{(i-1)}[1:i-1,i+1:n] +\\
&\mymidline{+ \underbrace{L^{(i-1)}[i+1:n,i]}_{=0}\cdot R^{(i-1)}[i,i+1:n] +}\LHS\\
&\mylastline{+ \underbrace{L^{(i-1)}[i+1:n,i+1:n]}_{=1_{n-i}}\cdot R^{(i-1)}[i+1:n,i+1:n]}\LHS\\
&= A[i+1:n,i+1:n].
\end{align*}
\end{document}

If you have the numbered version align*
you should also consider the width of the equation label and the label separator in the second argument of \mymidline
and \mylastline
.
I didn't find a predefined measure for the label width. After some tests it turned out that \quad\quad(1)
is an appropriate placeholder for the label.
\documentclass{article}
\usepackage[DIV15]{typearea}
\usepackage{amsmath,amsfonts}
\usepackage{fleqn}
\usepackage{ulem}
\makeatletter
\newdimen\@tzadima
\newdimen\@tzadimb
\newbox\@tzaboxa
\def\mylinemeasures#1#2{%
\@tzadima\displaywidth%
\advance\@tzadima-\tagwidth@%
\advance\@tzadima-\alignsep@%
\setbox\@tzaboxa\hbox{$\displaystyle#1$}%
\@tzadimb\wd\@tzaboxa%
\advance\@tzadima-\@tzadimb%
\setbox\@tzaboxa\hbox{$\displaystyle#2$}%
\@tzadimb\wd\@tzaboxa%
\advance\@tzadima-\@tzadimb%
}
\def\mymidline#1#2{%
\mylinemeasures{#1}{#2}%
\divide\@tzadima2%
\hbox to \@tzadima{}#1\notag
}
\def\mylastline#1#2{%
\mylinemeasures{#1}{#2}%
\hbox to \@tzadima{}#1%
}
\makeatother
\begin{document}
\begin{align}
\gdef\LHS{(L\cdot R)^{(i)}[i+1:n,i+1:n]}\LHS
&= \underbrace{L^{(i-1)}[i+1:n,1:i-1]\cdot R^{(i-1)}[1:i-1,i+1:n]}_{\text{untouched}}+\notag\\
&\mymidline{+ L^{(i)}[i+1:n,i] \underbrace{R^{(i)}[i,i+1:n]}_{\text{untouched pivot row}}+}\LHS\\
&\mylastline{+ \underbrace{L^{(i)}[i+1:n,i+1:n]}_{=1_{n-i-1}}\cdot R^{(i)}[i+1:n,i+1:n]}{\LHS\quad\quad(1)}\\
&= L^{(i-1)}[i+1:n,1:i-1]\cdot R^{(i-1)}[1:i-1,i+1:n] +\notag\\
&\mymidline{+ \uwave{L^{(i)}[i+1:n,i]\cdot R^{(i-1)}[i,i+1:n]}+}{(L\cdot R)^{(i)}[i+1:n,i+1:n]}\\
&\mylastline{+R^{(i-1)}[i+1:n,i+1:n]\uwave{\strut- L^{(i)}[i+1:n,i]\cdot R^{(i-1)}[i,i+1:n]}}{\LHS\quad\quad(1)}\\
&=L^{(i-1)}[i+1:n,1:i-1]\cdot R^{(i-1)}[1:i-1,i+1:n] +\notag\\
&\mymidline{+ \underbrace{L^{(i-1)}[i+1:n,i]}_{=0}\cdot R^{(i-1)}[i,i+1:n] +}{(L\cdot R)^{(i)}[i+1:n,i+1:n]}\\
&\mylastline{+ \underbrace{L^{(i-1)}[i+1:n,i+1:n]}_{=1_{n-i}}\cdot R^{(i-1)}[i+1:n,i+1:n]}{\LHS\quad\quad(1)}\\
&= A[i+1:n,i+1:n].
\end{align}
\end{document}

!
in front of it to turn it into a link. A moderator or another user with edit privileges can then reinsert the!
to turn it into an image again.\documentclass
and the appropriate packages so that those trying to help don't have to recreate it.split
for single long equations rather thanalign
. according to the document,split
is for single long equations,align
is for multiple equations. The numbering should be affected.