\documentclass{amsart}
\usepackage[all, ps, dvips]{xy} % Loading the XY-Pic package
                                % Using postscript driver for smoother
                                % curves
\usepackage{color}              % For invisible frame
\DeclareMathOperator{\SL}{SL}
\DeclareMathOperator{\PSL}{PSL}
\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@R=6pt{
  & B_3  \ar@{->>}[r]  \ar@{^(->}[dd]
  & \PSL_2(\mathbf{Z}) \ar@{^(->}[dd]\\
\mathbf{Z} \ar@{_(->}@/^/[ur] \ar@{^(->}@/_/[dr] \\
  & \overline{\SL_2(\mathbf{R})} \ar@{->>}[r]
  & \PSL_2(\mathbf{R})
}

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