\documentclass{book}
\usepackage[all]{xy}
\usepackage{amsthm,amssymb,amsmath,mathrsfs,amsfonts}
\begin{document}
\begin{equation}
\int d^3\vec{q}_2 d^3\vec{q}_2 \mathit{f}_{2,3}
\end{equation}

\begin{displaymath}\label{e520}
 b_2=\xymatrix{\bullet\ar@{.}[r]&\bullet  }=\int d^3\vec{q}_1 d^3\vec{q}_2 \mathit{f}(\vec{q}_1-\vec{q}_2)
\end{displaymath}

\begin{equation}
\int d^3\vec{q}_2 d^3\vec{q}_2 \mathit{f}_{2,3}
\end{equation}

\begin{displaymath}\label{e521}
 b_2=\xymatrix{\bullet\ar@{.}[r]&\bullet  }=\int d^3\vec{q}_1 d^3\vec{q}_2 \mathit{f}(\vec{q}_1-\vec{q}_2)
\end{displaymath}

\end{document}