# What is the matter with this equation array?

This seems to work just fine, but generates all sorts of errors.

$\left( \begin{array}{ccccccccc}1 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 1 & 1 & 1 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 \\1 & 0 & 0 & 1 & 0 & 0 & 1 & 0 & 0 \\0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 & 0 \\0 & 0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 \\0 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 \\0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 & 0 \\0 & 1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 1 & 0 & 1 & 0\end{array} \right) \times \left( \begin{array}{c}x_1 & x_2 & x_3 & x_4 & x_5 & x_6 & x_7 & x_8 & x_9\end{array} \right) =\left( \begin{array}{c}3 & 4 & 5 & 3 & 6 & 3 & 2 & 5 & 2 & 3 & 6 & 1\end{array} \right)$
• Welcome to TeX.SX! What kind of errors? Please help us help you and add a minimal working example (MWE) that illustrates your problem. Reproducing the problem and finding out what the issue is will be much easier when we see compilable code, starting with \documentclass{...} and ending with \end{document}. – albert Aug 23 at 14:47
• Your second (and third) \begin{array} is declared with a single column ({c}) but you are using 9 columns (x_1 & x_2 & ...). You need to either declare more columns, as in the first array, or replace the & by \\. – Phelype Oleinik Aug 23 at 14:51
• You cannot have blank lines within an equation. – Steven B. Segletes Aug 23 at 14:53
• Furthermore, mathematically, these matrices can't be multiplied – Bernard Aug 23 at 14:56

You should use \\ in the column vectors to separate entries (& separates entries in the same row).

Besides, \begin{equation}\end{equation} is out of place.

I suggest to use pmatrix, rather than array. My personal preference for matrices is bmatrix, though.

For unnumbered equations use equation*, for numbered ones use equation.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

Unnumbered version:
\begin{equation*}$$\begin{pmatrix} 1 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 & 1 & 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 \\ 1 & 0 & 0 & 1 & 0 & 0 & 1 & 0 & 0 \\ 0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 & 0 \\ 0 & 0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 \\ 0 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\ 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 \\ 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 & 0 \\ 0 & 1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 1 & 0 & 1 & 0 \end{pmatrix} \begin{pmatrix} x_1 \\ x_2 \\ x_3 \\ x_4 \\ x_5 \\ x_6 \\ x_7 \\ x_8 \\ x_9 \end{pmatrix} = \begin{pmatrix} 3 \\ 4 \\ 5 \\ 3 \\ 6 \\ 3 \\ 2 \\ 5 \\ 2 \\ 3 \\ 6 \\ 1 \end{pmatrix}$$\end{equation*}

Numbered version:
$$\begin{pmatrix} 1 & 1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 & 1 & 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 & 1 \\ 1 & 0 & 0 & 1 & 0 & 0 & 1 & 0 & 0 \\ 0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 & 0 \\ 0 & 0 & 1 & 0 & 0 & 1 & 0 & 0 & 1 \\ 0 & 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\ 1 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 \\ 0 & 0 & 0 & 1 & 0 & 0 & 0 & 1 & 0 \\ 0 & 1 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 1 & 0 & 1 & 0 \end{pmatrix} \begin{pmatrix} x_1 \\ x_2 \\ x_3 \\ x_4 \\ x_5 \\ x_6 \\ x_7 \\ x_8 \\ x_9 \end{pmatrix} = \begin{pmatrix} 3 \\ 4 \\ 5 \\ 3 \\ 6 \\ 3 \\ 2 \\ 5 \\ 2 \\ 3 \\ 6 \\ 1 \end{pmatrix}$$

\end{document}