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答案1
欢迎!tikz-cd
这很容易。
\documentclass[fleqn]{article}
\usepackage{tikz-cd}
\begin{document}
\[\begin{tikzcd}[column sep=small] 1\arrow[r] & 2 \arrow[r] & 3\arrow[ll,bend left]
\end{tikzcd}\]
You can control all aspects, e.g.\
\[\begin{tikzcd}[column sep=small] 1\arrow[r] & 2 \arrow[r] & 3
\arrow[ll,bend left=20,start anchor=-120,end anchor=-60]
\end{tikzcd}\]
\end{document}