6

I want to insert a Feynman diagram into an equation, but when I do so, I find that the Feynman diagram and the rest of the equation are awkwardly aligned. The picture should make it pretty clear what I mean. Is there a way to center the expression in this equation with the Feynman diagram?

enter image description here

\documentclass{article}

\usepackage{amsmath}
\usepackage{feynmf}

\begin{document}

\begin{fmffile}{test}
\begin{equation}
\begin{fmfgraph*}(65,50)
\fmfleft{i1,i2}
\fmfright{o1,o2}
\fmf{plain}{i1,v,o1}
\fmf{plain}{i2,v,o2}
\end{fmfgraph*}=-i\lambda
\end{equation}
\end{fmffile}

\end{document}
8

Use gathered:

\documentclass{article}

\usepackage{amsmath}
\usepackage{feynmp-auto} % or feynmf

\begin{document}

\begin{fmffile}{test}
\begin{equation}
\begin{gathered}
\begin{fmfgraph*}(65,50)
\fmfleft{i1,i2}
\fmfright{o1,o2}
\fmf{plain}{i1,v,o1}
\fmf{plain}{i2,v,o2}
\end{fmfgraph*}
\end{gathered}=-i\lambda
\end{equation}
\end{fmffile}

\end{document}

enter image description here

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