\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}$$% Frame for margin\xymatrix@=6pt{ &&K^{n+1} \ar@{->>}[dd]\\ && \\\mathbf{A}^n \ar@{^(->}[rr] &&\mathbf{P}^n}$$}}}% end math, end frame\end{document}