A commutative diagram defining equivariance. David Benbennick made this image with the following LaTeX program:

%&latex
\documentclass{article}
 \usepackage[matrix,arrow,ps]{xy}
 \UsePSspecials{dvips}
 \pagestyle{empty}

\begin{document}
 \[][rr]^{\displaystyle g\cdot} \ar@{->}[dd]_{\displaystyle f}
      && X \ar@{->}[dd]^{\displaystyle f}    \\ \\
 Y \ar@{->}[rr]_{\displaystyle g\cdot} && Y
 }
 \]
\end{document}

To make the image, save the above code as Equivariant_commutative_diagram.tex, then run the following commands:

tex Equivariant_commutative_diagram
dvips -mode ljfzzz -D 1200 Equivariant_commutative_diagram -o
convert -density 1200 Equivariant_commutative_diagram.ps Equivariant_commutative_diagram.pnm
pnmcrop Equivariant_commutative_diagram.pnm | pnmtopng > Equivariant_commutative_diagram.png.uncrush
pngcrush Equivariant_commutative_diagram.png.uncrush Equivariant_commutative_diagram.png