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答案1
\documentclass{article}
\usepackage{amsmath}
\usepackage[all]{xy}
\begin{document}
\[
\xymatrix{
1\ {\circ} \ar@{}@/^/[r] \ar@{}@/_/[r]
& {\circ}\ 1
}
\]
\end{document}
\documentclass{article}
\usepackage{amsmath}
\usepackage[all]{xy}
\begin{document}
\[
\xymatrix{
1\ {\circ} \ar@{}@/^/[r] \ar@{}@/_/[r]
& {\circ}\ 1
}
\]
\end{document}