\documentclass{amsart}
\usepackage[all, ps]{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@=6pt{
   A   \ar[rr]               \ar@{^(->}[dd]
 && \mathbf{K}^*             \ar@{^(->}^{\text{diag}}[dd]
\\ \\
   C   \ar^(.3){\sigma}[rr]  \ar@{->>}^\phi[dd] 
 &&\operatorname{GL}(n,\mathbf{K})  \ar@{->>}^\pi[dd]
\\
 &&
\\
   G   \ar^(.3){\rho}[rr]
 &&\operatorname{PGL}(n,\mathbf{K})
}
 
$$}}} % end math, end frame
\end{document}