`\multicolumn` funktioniert auch in `array`.
\documentclass[border=3mm]{standalone}
%\documentclass{article}
\usepackage[ngerman]{babel}
\usepackage{amsmath}
\begin{document}
$
\begin{array}{lllc}
z_0 & z_n = z_0 & z_{2n} = z_n = z_0 & \ldots \\
z_1 & z_{n+1} = z_1 & z_{2n+1} = z_{n+1} = z_1 & \ldots \\
z_2 & z_{n+2} = z_2 & z_{2n+2} = z_{n+2} = z_2 & \ldots \\
\multicolumn{1}{c}{$\vdots$} & \multicolumn{1}{c}{$\vdots$} & \multicolumn{1}{c}{$\vdots$} & \vdots \\
z_{n-1} & z_{2n-1} = z_{n-1} & z_{3n-1}=z_{2n-1}=z_{n-1}& \ldots
\end{array}
$
\end{document}
[![alt text][1]][1]
[1]: http://texwelt.de/wissen/upfiles/test_366.png<a href="http://texwelt.de/wissen/upfiles/test_366.png">
<img src="http://texwelt.de/wissen/upfiles/test_366.png" style="max-width:640px;"/>
</a>
