# How to make two 2x1 matrices in the same column?

I'm using LaTeX to show the butterfly visualization of fast Fourier transformation and I'm stuck at drawing the matrix.

I want something like this:

i.e. if I have a vector of length 8, in the next column (after the equal sign), I want two matrices each of length 4 on the same column one below the other, how can I achieve this?

\documentclass{article}
\begin{document}

\left[ \begin{array}{ccc}
1 \\
0 \\
0 \\
0 \\
3 \\
0 \\
0 \\
0 \\
\end{array} \right]

\end{document}

• Welcome to TeX.SX! Please help us to help you and add a minimal working example (MWE) that illustrates your problem. It will be much easier for us to reproduce your situation and find out what the issue is when we see compilable code, starting with \documentclass{...} and ending with \end{document}. – user31729 Jul 12 '14 at 16:16

This requires some manual adjustment, but if you don't have too many of these butterflies, it might be sufficient:

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

\ExplSyntaxOn
\RenewDocumentCommand{\vdots}{O{3}}
{
\vbox:n
{
\skip_set:Nn \baselineskip {4pt}
\dim_set:Nn \lineskiplimit {0pt}
\kern 6pt % no equivalent with expl3 at the moment
\prg_replicate:nn { #1 } { \hbox:n { . } }
}
}
\ExplSyntaxOff

\begin{document}
$\begin{bmatrix} 0 \\ \vdots[16] \\ n \end{bmatrix} = \begin{matrix} \begin{bmatrix} 0 \\ \vdots \\ (n+1)/2 \end{bmatrix} \\[4.7ex] \begin{bmatrix} 0 \\ \vdots \\ (n+1)/2 \end{bmatrix} \end{matrix}$
\end{document}


The new optional argument to \vdots tells how many dots are to be printed.

• nice solution. However, it does not work properly when the matrices are NOT one-column matrices. Here more details in a question. Any idea on how to solve the issue? – Leos313 Apr 30 at 16:41
\documentclass[preview,border=12pt,12pt]{standalone}
\usepackage{amsmath}

\begin{document}

\begin{align*}
\begin{bmatrix}
a\\b\\c\\d\\e\\f\\g\\h
\end{bmatrix}
&=\!\begin{aligned}
&\begin{bmatrix}a\\b\\c\\d\end{bmatrix}\\
&\begin{bmatrix}a\\b\\c\\d\end{bmatrix}
\end{aligned}\\
&=\ldots
\end{align*}

\end{document}


## More practical example

\documentclass[preview,border=12pt,12pt]{standalone}
\usepackage{amsmath}

\begin{document}

\begin{align*}
\begin{bmatrix}
0\\1\\2\\3\\\vdots\\n-2\\n-1\\n
\end{bmatrix}
&=\!\begin{aligned}
&\begin{bmatrix}0\\1\\\vdots\\\frac{n+1}{2}\end{bmatrix}\\
&\begin{bmatrix}0\\1\\\vdots\\\frac{n+1}{2}\end{bmatrix}
\end{aligned} + \text{ what ever \ldots}\\
&=\!\begin{aligned}
&\begin{bmatrix}0\\1\\\vdots\\\frac{n+1}{2}\end{bmatrix}\\
&\begin{bmatrix}0\\1\\\vdots\\\frac{n+1}{2}\end{bmatrix}
\end{aligned} + \text{ forever \ldots}
\end{align*}

\end{document}