# Equation number for each row of a matrix equation

Is it possible to give an equation number to each row of a matrix equation?

How could I have equation labels in the vein of subequation numbers, one for each row of the matrix equation? Here, I'd like equation numbers such as, e.g., (3.1a), (3.1b), (3.1c) (I don't mean row/column labels as in a bordered matrix).

Thanks!

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Use an image we don't use mathjax on this site as it would confuse the issue as people need to show the tex rendering amd mathjax does not use tex –  David Carlisle May 9 '14 at 23:36
Related, but not automatic numbering: Label rows of a matrix by characters –  Mike Renfro May 9 '14 at 23:39
Thanks. I placed an image. –  Dominique May 9 '14 at 23:42
I can't understand what the line number would refer to. –  egreg May 10 '14 at 7:44
@egreg each line number would refer to a block equation of the system. –  Dominique May 10 '14 at 19:44

Hmm! not really supported LaTeX syntax but

\documentclass{article}

\usepackage{amsmath}

\newsavebox\labelbox

\begin{document}

\savebox\labelbox{$\begin{matrix} \refstepcounter{equation}(\theequation)\label{aa}\\ \refstepcounter{equation}(\theequation)\label{bb}\\ \refstepcounter{equation}(\theequation)\label{cc} \end{matrix}$}

$\begin{bmatrix} H & J^T & -Z^{\frac12} \\ J & -\delta I & \\ -Z^{\frac12} & & -X \end{bmatrix} \begin{bmatrix} r \\ s \\ t \end{bmatrix} = \lambda \begin{bmatrix} H & & \\ & U & \\ & & W \end{bmatrix} \begin{bmatrix} r \\ s \\ t \end{bmatrix} \eqno \usebox{\labelbox}$

[\ref{aa}]
[\ref{bb}]
[\ref{cc}]
\end{document}

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Very interesting (and potentially useful) technique. Is there anything that can go wrong with it? –  Steven B. Segletes May 10 '14 at 2:01
I didn't think of that. I like it! Why do you say it's not supported?It's not inelegant. –  Dominique May 10 '14 at 4:30
\eqno is a TeX primitive and wouldn't generally work in conjunction with amsmath constructs, but you get away with it here. Probably you could (and should) use \tag in other contexts. –  David Carlisle May 10 '14 at 10:04
@StevenB.Segletes something can go wrong with anything:-) \eqno shouldn't be used in latex (and wouldn't work in most ams math constructs) but it works here and I was getting tired:-) –  David Carlisle May 10 '14 at 10:06
But you know me. I'm not contemplating using in with ams math constructs, but rather with TABstacks ;^) –  Steven B. Segletes May 10 '14 at 11:10

Below there is a possible quick and dirty alternative if one needs AMS-\tags. The drawback is that it requires some manual height adjustment in the first matrix. This solution would not exist without the solution of David. Thanks, David! I needed this too.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\par\noindent\begin{minipage}[c][][c]{0.9\textwidth}
\begin{align*}
\begin{matrix}
1\cdot x_1&+&2\cdot x_2&+&3\cdot x_3&=&4\\[3pt]
5\cdot x_1&+&6\cdot x_2&+&7\cdot x_3&=&8\\[3pt]
9\cdot x_1&+&10\cdot x_2&+&11\cdot x_4&=&12
\end{matrix}
\end{align*}
\end{minipage}\begin{minipage}[c][][c]{0.1\textwidth}
\begin{align}\refstepcounter{equation}
\tag{\theequation a}\label{eq:1a}\\[0pt]
\tag{\theequation b}\label{eq:1b}\\[0pt]
\tag{\theequation c}\label{eq:1c}
\end{align}
\end{minipage}
\end{document}

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