ctex问题

源文件如下
\documentclass[a4paper,12pt]{article}
\begin{document}
\ldots when Einstein introduced his formula
\begin{equation} \label{1}
e = m \cdot c^2 \;,
\end{equation}
which is at the same time the most widely known and
the least well understood physical formula
\end{document}

请问公式后面\;有什么用？

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\cdot 表示乘号， \；表示空格。

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没什么实际的用处只是一个小的间距
没有必要加这样的间距。

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