General ⇒ Numbering matrices

[msgj]

Can anyone tell me how I might number a matrix, line by line? I would like the numbers outside of the matrix delimiters, e.g. something like this:

| x x x |
| x x x | (2)
| x x x | (3)

If I use the equation environment with an array, then I will just get one number for the whole matrix. If I use an environment like align, there seems to be no way to get the matrix delimiters.

I have tried doing it manually with some code like this:

Code: Select all

\addtocounter{equation}{1}\newcounter{eqn}
\[C(\Gamma)=\left[\begin{array}{3}x_1&x_2&x_3\\
y_1&y_2&y_3\\
z_1&z_2&z_3
\end{array}\right]
\begin{array}{l}\\
(\theequation)\setcounter{eqn}{\theequation}\addtocounter{equation}{1}\\
(\theequation)\\
\end{array}\]

in which I put the numbers in a second array and increment the counter manually. (The numbers for the second and third lines are stored in the counters eqn and \theequation respectively.)

However, this is not a satisfactory solution, and does not work when I introduce chapters because then the line numbers become of the form 2.13 which are not incremented so easily.

If there is an environment which will do this for me, please let me know!

Thanks.

[gmedina]

I don't know if some package directly offers you the solution to your problem. However, your approach seems more or less right. You can achieve the numbering in a simpler way using something like this

Code: Select all

\documentclass{article}
\usepackage{amsmath,array}

%***the new counter*******************************
\newcounter{mymatrow}
\stepcounter{mymatrow}

\begin{document}
\[
\left[
\begin{array}{cc}
  0 & 1\\
  0 & 1\\
  0 & 1
\end{array}
\right]
\hspace{5pt}
\begin{array}{l}\\
(\themymatrow)\stepcounter{mymatrow}\\
(\themymatrow)\\
\end{array}
\]

\end{document}

[msgj]

Thanks for the reply. But then ...

a) how would I refer to the first of the two labels? (i.e. how do i subtract 1 from mymatrow in the \ref command?)

b) these numbers would not follow other numbers in the document, and would not be in the style chapter.number, would they?

[gmedina]

Then you can try something like this:

Code: Select all

\documentclass{book}
\usepackage{amsmath,array}

\begin{document}

\chapter{First chapter}

\begin{center}
  \begin{minipage}{.3\linewidth}
    \begin{equation*}
    \renewcommand{\arraystretch}{1.25}
    \left[
    \begin{array}{cc}
      0 & 1\\
      0 & 1\\
      0 & 1
    \end{array}
    \right]
    \end{equation*}
  \end{minipage}\hspace{-2.5cm}
  \begin{minipage}{.2\linewidth}
  \vspace*{3pt}
    \begin{align}
    \ \label{mat:row1}\\
    \ \label{mat:row2}
    \end{align}
  \end{minipage}
\end{center}
As the rows \ref{mat:row1} and \ref{mat:row2} show,...

\end{document}

It is not an optimal solution, but at least you can cross-reference the rows of the matrix using the standard commands and you won't need to worry about new counters.

[Juanjo]

The above approaches can fail if the heights of the matrix rows are a bit different. For example, replace 1 by \dfrac{1}{2}: alignment between a row and the corresponding number is lost. So you should have to use \phantom and like. In addition, some adjustment is required to vertically align row labels with the remaining equation labels in the page.

Instead of searching a LaTeX solution for the problem, I wonder if it would be preferable to write things in a different way. From a mathematical standpoint, it is strange to number rows of a matrix instead of the whole equation. If I were a reader of the document, I would prefer to see something like "the second row of the matrix in (1.2)", instead of "the row (1.2)", which sounds rare.

[msgj]

Thanks for both of your suggestions. I will let you know how I get on.

Happily there are no fractions in the matrices, so hopefully vertical alignment won't be a problem. I take your point about referring to the lines in a matrix, but I still prefer to do it this way if I can! Actually they are not matrices but presentations of groups (I didn't want to make the question unnecessarily complicated.) Thus I don't want to refer to "the second line of the relations in the presentation above ..." but simply to "relations 1.2".

[msgj]

I attach a page from my thesis so you can see what I'm talking about. I'm using square brackets for the presentations rather than the traditional angled ones, to save space.