\documentclass{amsart}
\usepackage{amsmath,amssymb,nopageno}
\usepackage[all]{xy}
\begin{document}
 \begin{equation*}
  \xymatrix@C+2em@R+2em{
   P \ar@{<-}[d]_{i_1} \ar@{<-}[r]^{i_2} & Y \ar@{<-}[d]^{g}\\
   X \ar@{<-}[r]_{f} & Z
  }
 \end{equation*}
\end{document}

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