# How to draw the calculation of a determinant?

I'm trying to reproduce the following calculation of a determinant in latex:

Any ideas? I thought of using the tabular environment but unfortunately I'm not able to align the "=" correctly.

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Please, add a Minimal Working Example, so the people could easily think and try with your problem. –  Manuel Jan 11 '13 at 20:01

I just have one question: The numbers in the determine itself are all right aligned. You can see it in column 3, row 1 and 2 in the picture I posted. The 2 is just above the -2 and they are both right aligned, no matter if there is a minus or not. Do you have any idea how to realize that? -- user24295

No, I don't, but according to comments by percusse and Manuel you can do that by loading mathtools instead of amsmath and use \begin{vmatrix*}[r] … \end{vmatrix*}. I haven't checked though by myself. But I do recomment keep the standard alignement provided by \begin{vmatrix} ... \end{vmatrix}.

UPDATE with align. Thanks to cmhughes's advice I've replaced eqnarray with alignand &=& with &=. ADDED. As commented by egreg eqnarray should be avoided (see e.g. here and here) due to spacing discrepancies.

The updated code is:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
\det A &=\left.
\begin{array}{c}
\;\;\text{*)} \\
-1\text{)} \\
-2\text{)} \\
-2\text{)}
\end{array}
\right.
\begin{vmatrix}
1 & 3 & 2 & -6 \\
1 & 2 & -2 & -5 \\
2 & 4 & -2 & -9 \\
2 & 4 & -6 & -9  \notag
\end{vmatrix}
=
\begin{vmatrix}
1 & 3 & 2 & -6 \\
0 & -1 & -4 & 1 \\
0 & -2 & -6 & 3 \\
0 & -2 & -10 & 3
\end{vmatrix}
\\ \notag
&& \\
&=\left.
\begin{array}{c}
\;\;\text{*)} \\
-2\text{)} \\
-2\text{)}
\end{array}
\right.
\begin{vmatrix}
1 & 4 & 1 \\
2 & 6 & 3 \\
2 & 10 & 3
\end{vmatrix}
=
\begin{vmatrix}
1 & 4 & 1 \\
0 & -2 & 1 \\
0 & 2 & 1
\end{vmatrix}
=
\begin{vmatrix}
-2 & 1 \\
2 & 1
\end{vmatrix}
=-4\notag
\end{align}

\end{document}


The Output is:

We can use the following code (with eqnarray)

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{eqnarray*}
\det A\; &=&\left.
\begin{array}{c}
\;\;\text{*)} \\
-1\text{)} \\
-2\text{)} \\
-2\text{)}
\end{array}
\right.
\begin{vmatrix}
1 & 3 & 2 & -6 \\
1 & 2 & -2 & -5 \\
2 & 4 & -2 & -9 \\
2 & 4 & -6 & -9
\end{vmatrix}
=
\begin{vmatrix}
1 & 3 & 2 & -6 \\
0 & -1 & -4 & 1 \\
0 & -2 & -6 & 3 \\
0 & -2 & -10 & 3
\end{vmatrix}
\\
&& \\
&=&\left.
\begin{array}{c}
\;\;\text{*)} \\
-2\text{)} \\
-2\text{)}
\end{array}
\right.
\begin{vmatrix}
1 & 4 & 1 \\
2 & 6 & 3 \\
2 & 10 & 3
\end{vmatrix}
=
\begin{vmatrix}
1 & 4 & 1 \\
0 & -2 & 1 \\
0 & 2 & 1
\end{vmatrix}
=
\begin{vmatrix}
-2 & 1 \\
2 & 1
\end{vmatrix}
=-4
\end{eqnarray*}

\end{document}


to type

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Please, don't recommend eqnarray under any circumstances. –  egreg Jan 11 '13 at 20:26
Very nice except the eqnarray. Here is one out of many reasons. (vote capped for today) –  percusse Jan 11 '13 at 20:29
s/eqnarray/align/g and s/&=&/&=/g should do the trick –  cmhughes Jan 11 '13 at 20:35
Very nice done. Thank you @Américo Tavares for your help. I just have one question: The numbers in the determine itself are all right aligned. You can see it in column 3, row 1 and 2 in the picture I posted. The 2 is just above the -2 and they are both right aligned, no matter if there is a minus or not. Do you have any idea how to realize that? –  Martin Jan 11 '13 at 20:39
@AméricoTavares To right align the columns you could use \begin{vmatrix*}[r] … \end{vmatrix*}. –  Manuel Jan 11 '13 at 21:29

You can define an xvmatrix environment for the "eXtended" matrix, where you specify the coefficients as a first column:

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newenvironment{xvmatrix}% eXtended vmatrix
{\left.\array{@{}r |@{\,}*\c@MaxMatrixCols c}}
{\endarray\kern-\arraycolsep\right|}

\begin{document}

\begin{align*}
\det A &=
\begin{xvmatrix}
*) & 1 & 3 &  2 & -6 \\
-1) & 1 & 2 & -2 & -5 \\
-2) & 2 & 4 & -2 & -9 \\
-2) & 2 & 4 & -6 & -9
\end{xvmatrix}
=
\begin{vmatrix}
1 & 3 & 2 & -6 \\
0 & -1 & -4 & 1 \\
0 & -2 & -6 & 3 \\
0 & -2 & -10 & 3
\end{vmatrix}
\\
&=
\begin{xvmatrix}
*) & 1 &  4 & 1 \\
-2) & 2 &  6 & 3 \\
-2) & 2 & 10 & 3
\end{xvmatrix}
=
\begin{vmatrix}
1 & 4 & 1 \\
0 & -2 & 1 \\
0 & 2 & 1
\end{vmatrix}
=
\begin{vmatrix}
-2 & 1 \\
2 & 1
\end{vmatrix}
=-4
\end{align*}

\end{document}


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And if that's always the notation, you could add ) to the environment so you don't need to write it all the times. Apart from that, I don't find this notation pleasant (*), etc. on the left of the matrix), I don't suggest any other, but I think this is not beautiful. –  Manuel Jan 11 '13 at 21:27
@Manuel I thought about adding the ) to the definition, but maybe leaving it in the cell makes clearer what the number is for. I don't like the notation, either. There's no need to use the Laplace development at any stage, the usual Gaussian elimination suffices. –  egreg Jan 11 '13 at 21:30
Nitpicking: In the OP's picture, the numbers are right aligned. –  mafp Jan 12 '13 at 1:23
@mafp I never right align the coefficients of a matrix. Adding that feature is possible, though. –  egreg Jan 12 '13 at 10:45
@egreg Oh, I would not write the whole determinant computation that way. But it is what the OP asked for. It works with mathtools, and then vmatrix[r], right? –  mafp Jan 12 '13 at 13:12

I do not consider a good habit introducing such a notation, my experience is that it only confuses people. Mathematical formalism is there for a reason. You can try the following:

\documentclass{article}
\pagestyle{empty}
\usepackage{amsmath}
\begin{document}

\begin{align*}
\det A&=
\underbrace{\begin{vmatrix}
1 & 0 & 0 & 0 \\
-1 & 1 & 0 & 0 \\
-2 & 0 & 1 & 0 \\
-2 & 0 & 0 & 1
\end{vmatrix}}_{=1}
\cdot
\begin{vmatrix}
1 & 3 & 2 & -6 \\
1 & 2 & -2 & -5 \\
2 & 4 & -2 & -9 \\
2 & 4 & -6 & -9
\end{vmatrix}
=
\begin{vmatrix}
1 & 3 & 2 & -6 \\
0 & -1 & -4 & 1 \\
0 & -2 & -6 & 3 \\
0 & -2 & -10 & 3
\end{vmatrix}
\\
&=
\underbrace{\begin{vmatrix}
1 & 0 & 0 \\
-2 & 1 & 0 \\
-2 & 0 & 1
\end{vmatrix}}_{=1}
\cdot
\begin{vmatrix}
1 & 4 & 1 \\
2 & 6 & 3 \\
2 & 10 & 3
\end{vmatrix}
=
\begin{vmatrix}
1 & 4 & 1 \\
0 & -2 & 1 \\
0 & 2 & 1
\end{vmatrix}
=
\begin{vmatrix}
-2 & 1 \\
2 & 1
\end{vmatrix}
=-4
\end{align*}

\end{document}


Somehow, it would make more sense if you tried to demonstrate the column manipulation, because then the triangle unit-determinant matrix would come on the right side. But even this is completely clear IMHO.

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