Created as per: en:meta:Help:Displaying a formula#Commutative diagrams

% \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}$$ % Frame for margin
% 
% \xymatrix{
% (M \otimes M) \otimes M \ar[d]_{\mu \otimes 1} 
% \ar[rr]^{\alpha} & & M \otimes (M \otimes M) \ar[d]^{1 
% \otimes \mu} \\
% M \otimes M \ar[r]_{\mu} & M & M \otimes M \ar[l]^{\mu}
% }
% 
% $$}}} % end math, end frame
% \end{document}