Amsmath：当名字很长时使用 \tag{name}

Question

当名称很长时，即使标记的名称不会与内容重叠，方程式的居中也会偏离。如何将名称较长的方程式居中？

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
  1 &= 1\\
  2 &= 2\tag{This is a long tag}\\
  3 &= 3
\end{align*}
\end{document}

从下图中，我们可以看到有足够的空间让方程式与文档居中并且不会重叠。

Answer 1

一个选项是使用 ; 等来隐藏标签宽度\makebox，在Dequation环境中，标签宽度将不予考虑：

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newenvironment{Dequation}
  {%
  \def\tagform@##1{%
    \maketag@@@{\makebox[0pt][r]{(\ignorespaces##1\unskip\@@italiccorr)}}}%
  \ignorespaces
  }
  {%
  \def\tagform@##1{\maketag@@@{(\ignorespaces##1\unskip\@@italiccorr)}}%
  \ignorespacesafterend
  }
\makeatother    

\begin{document}

A regular \texttt{align*} with long tags:
\begin{align*}
  1 &= 1\\
  2 &= 2\tag{This is a long tag}\\
  3 &= 3\tag{This is an even longer tag}
\end{align*}
A regular \texttt{align*} without tags, for comparison:
\begin{align*}
  1 &= 1\\
  2 &= 2\\
  3 &= 3
\end{align*}
An \texttt{align*} with long tags inside \texttt{Dequation}:
\begin{Dequation}
\begin{align*}
  1 &= 1\\
  2 &= 2\tag{This is a long tag}\\
  3 &= 3\tag{This is an even longer tag}
\end{align*}
\end{Dequation}
The initial regular \texttt{align*} with long tags, for comparison:
\begin{align*}
  1 &= 1\\
  2 &= 2\tag{This is a long tag}\\
  3 &= 3\tag{This is an even longer tag}
\end{align*}

\end{document}

Answer 2

您可以在另一侧添加最长的标签，以\phantom“恢复”水平偏移：

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align*}
  1 &= 1 \\
  2 &= 2 \\
  3 &= 3
\end{align*}
\begin{align*}
  1 &= 1 \\
  2 &= 2 \tag{This is a long tag} \\
  3 &= 3
\end{align*}
\begin{align*}
  1 &= 1 \\
  \phantom{\text{(This is a long tag)}}2 &= 2 \tag{This is a long tag} \\
  3 &= 3
\end{align*}
\end{document}