I want to write the companion matrix of a recurrence relation and its use (the notation will be a bit unusual since it is "for cryptography", where we always do stuff in a different way).
\documentclass[11pt]{article}
\usepackage{amsmath}
\begin{document}
\[
C :=
\begin{pmatrix}
0 & 1 & 0 & \cdots & 0 \\
0 & 0 & 1 & \cdots & 0 \\
\vdots & \vdots & \vdots & \ddots & \vdots \\
0 & 0 & 0 & \ddots & 1 \\
c_0 & c_1 & c_2 & \cdots & c_{n-1} \\
\end{pmatrix}
%%%
\quad\textrm{and}\quad
%%%
C \cdot
\begin{pmatrix}
y_{k} \\ y_{k+1} \\ \vdots \\ y_{n+k-2} \\ y_{n+k-1} \\
\end{pmatrix}
=
\begin{pmatrix}
y_{k+1} \\ y_{k+2} \\ \vdots \\ y_{n+k-1} \\ y_{n+k} \\
\end{pmatrix}
\enspace.
\]
\end{document}
however, the square matrix is taller than the column vectors. How can I make everything align nicely?
Roberto
EDIT: Thank you all folks, in my case it was the wrong \ddots (ahem).
EDIT 2:
So, merging @percusse's suggestion with braces over matrix I came up with this
\def\matriximg{%
\begin{array}{c|ccc}
\\
\vphantom{\vdots} 0 &\multicolumn{3}{c}{I_{n-1}} \\
\\ \hline
\vphantom{y_k} c_0 & c_1 & \ldots &c_{n-1}
\end{array}
}%
\begin{equation}
\label{eq:companion}
\left(\vphantom{\matriximg}\right.%
\kern-2\nulldelimiterspace%
\underbrace{\matriximg}_{\displaystyle C}%
\kern-\nulldelimiterspace%
\left.\vphantom{\matriximg}\right)%
%
\cdot
%
\left(\!\begin{array}{c}
y_{k} \\ \hline y_{k+1} \\ \vdots \\ y_{n+k-1} \\
\end{array}\!\right)
=
\left(\!\begin{array}{c}
y_{k+1} \\ \vdots \\ y_{n+k-1} \\ \hline y_{n+k} \\
\end{array}\!\right)
\enspace.
\end{equation}
\cdots
, all problems are gone.