\documentclass{article}
\usepackage{xy}
\xyoption{all}
\begin{document}
\pagestyle{empty}
\[
\xymatrix{
& K \otimes K \cong K \ar[ld]_{\eta \otimes \eta} \ar[rd]^\eta & \\
B \otimes B  & & B \ar[ll]^\Delta \\
}
\]
\end{document}

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