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# 双重下标

## 简介

按照惯例，LaTeX 使用下划线字符（`_`）来排版下标，例如 `$a_b$` 其效果是 $$a\_b$$。如果你写 `$a_bcde$` 只有第一个字符， `b`，会成为下标， $$a\_bcde$$，因为 LaTeX 不会 *假定* 你想把整组字符都设为下标 `bcde`。要将多个项目（标记符）排版为下标，你需要通过将它们括在一组中来告诉 LaTeX `{...}`，像这样： `$a_{bcde}$`，其效果是 $$a\_{bcde}$$.

## 双重下标错误

当要求 LaTeX 为某个已经 *已经有* 附加了下标的数学内容再添加一个下标时，就会出现双重下标错误。这个错误通常可通过使用花括号来解决 `{...}` 在数学模式中，它们会生成所谓的 *子公式*——用于描述你试图排版的数学表达式片段的术语。

例如，写作 `$a_b_c$` 会产生如下双重下标错误：

![Overleaf 上显示的双重下标错误](/files/bfef6beef0881cfc8fa086865cf0ed3e70e07d0d)

这个特定错误可以通过使用花括号以多种方式修复 `{...}`——结果会因花括号的位置不同而异：

* `$a_{b_c}$` 排版为 $$\displaystyle a\_{b\_c}$$
* `${a_b}_c$` 排版为 $${a\_b}\_c$$
* `$a_{bc}$` 排版为 $$a\_{bc}$$

### 双重下标错误：一个特殊情况

在以下网站上的一个问题 `tex.stackexchange` 展示了 [一个令人惊讶的双重下标错误示例](https://tex.stackexchange.com/questions/253080/why-am-i-getting-a-double-subscript-error)，其原因及相应修复方法值得了解。我们在此重现该示例。

尽管使用了花括号，下面的示例仍无法编译：

```latex
\documentclass{article}
\begin{document}
\({\vec a_b}_c\)
\end{document}
```

[打开此 ***会产生错误的*** Overleaf 中的示例](https://www.overleaf.com/docs?engine=pdflatex\&snip_name=accents+package+example\&snip=%5Cdocumentclass%7Barticle%7D%0A%5Cbegin%7Bdocument%7D%0A%5C%28%7B%5Cvec+a_b%7D_c%5C%29%0A%5Cend%7Bdocument%7D)

解决此问题的一种方法是添加 [`accents` 宏包](https://ctan.org/pkg/accents?lang=en) （如 [此处建议](https://tex.stackexchange.com/a/253094)):

```latex
\documentclass{article}
\usepackage{accents}
\begin{document}
\({\vec a_b}_c\)
\end{document}
```

[在 Overleaf 中打开此示例](https://www.overleaf.com/docs?engine=pdflatex\&snip_name=accents+package+example\&snip=%5Cdocumentclass%7Barticle%7D%0A%5Cusepackage%7Baccents%7D%0A%5Cbegin%7Bdocument%7D%0A%5C%28%7B%5Cvec+a_b%7D_c%5C%29%0A%5Cend%7Bdocument%7D)

## 更多关于下标的内容

可以通过编写类似以下的 LaTeX 来排版更深层级的下标 `$a_{b_{c_{d_e}}}$` 其效果是排版为 $$a\_{b\_{c\_{d\_e}}}$$。第二层及以下的所有下标（即下下标）都使用相同的字体大小（以磅为单位）排版，而第一层下标则使用稍大一些的字体大小（磅值）排版。要了解更多关于下标的内容，请访问 Overleaf 的 [专门介绍该主题的帮助页面](/latex/zh-cn/shu-xue/02-subscripts-and-superscripts.md).


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