\documentclass[12pt]{article}
\usepackage{amsthm,amssymb,amsmath}
\usepackage[all]{xy}
\usepackage{pgf,pgfplots,tikz}
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\usepackage{xepersian}
\settextfont[Scale=1]{XB Niloofar}
\numberwithin{equation}{section}
\begin{document}
\section{اول}

\begin{center}
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\end{center}
\begin{equation}
\xymatrix{
                                                                               & \mathcal{U}_\alpha \ar[dr]^{I^\alpha}  &  \\
 \Sigma_g \ar[ur]^{i^\alpha} \ar[dr]_{i^\beta} &                                      & \mathcal{U}_\alpha \cup_\Sigma \mathcal{U}_\beta   \\
                                                                               &  \mathcal{U}_ \beta \ar[ur]^{I^ \beta}  &  }
\end{equation}
\end{document}