\documentclass{amsart}
\usepackage[all, ps, dvips]{xy} % Loading the XY-Pic package
                                % Using postscript driver for smoother curves
\usepackage{color}              % For invisible frame
\begin{document}
\thispagestyle{empty} % No page numbers
\SelectTips{eu}{}     % Euler arrowheads (tips)
\setlength{\fboxsep}{0pt} % Frame box margin
{\color{white}\framebox{{\color{black}$$ % Frame for margin

\xymatrix{ %Our diagram is a 3x3 matrix
\mathbf{Z}/n \ar@{^(->}[d]\ar@{^(->}[r]
  & U(1) \ar@{^(->}[d]\ar@{->>}[r]^{z^n}
  & U(1) \ar[d]^\wr\\
SU(n)    \ar@{->>}[d]\ar@{^(->}[r]
  & U(n) \ar@{->>}[d]\ar@{->>}[r]^{\det}
  & U(1)\\
PSU(n)   \ar[r]^\sim
  & PU(n)
}

$$}}} % end math, end frame
\end{document}