具有多个方程编号的多行环境

Question

我希望a拥有b自己的方程式编号，同时保持multline提供的整体布局。这可能吗？

\documentclass{article}
\usepackage{amsmath}
\begin{document}
    \begin{multline}
    a \\
    b \\
    c
    \end{multline}
\end{document}

Answer 1

这也许不能完成所有的检查multline，但可能足以满足您的目的：

\documentclass{article}
\usepackage{amsmath}
\usepackage{environ}
\usepackage{xparse}

\ExplSyntaxOn
\NewEnviron{nmultline}{%
  \seq_set_split:NnV \l_nmultline_contents_seq { \\ } \BODY
  \seq_pop_left:NN \l_nmultline_contents_seq \l_nmultline_first_tl
  \seq_pop_right:NN \l_nmultline_contents_seq \l_nmultline_last_tl
  \begin{gather}
  \makebox[\dim_eval:n{\displaywidth-4em}][l]{$\displaystyle \l_nmultline_first_tl$} \\
  \seq_use:Nn \l_nmultline_contents_seq { \\ } 
  \seq_if_empty:NF \l_nmultline_contents_seq { \\ }
  \makebox[\dim_eval:n{\displaywidth-4em}][r]{$\displaystyle \l_nmultline_last_tl$}
  \end{gather}
}
\seq_new:N \l_nmultline_contents_seq
\tl_new:N \l_nmultline_first_tl
\tl_new:N \l_nmultline_last_tl
\ExplSyntaxOff

\begin{document}
For comparison, first four rows
\begin{multline}
a \\
b \\
c \\
d
\end{multline}
and then two rows
\begin{multline}
a \\
b
\end{multline}
Now numbered:
\begin{nmultline}
a\label{a} \\
b\label{b} \\
c\label{c} \\
d\label{d}
\end{nmultline}
References: \eqref{a}, \eqref{c}, \eqref{bb}
\begin{nmultline}
a\label{aa} \\
b\label{bb}
\end{nmultline}

\end{document}

Answer 2

根据@egreg的回答，我得出了以下结论。它基本上定义了\gatherleft与\gatherright间距一致的命令\multline，即使方程编号（包括标签）的长度不同。

它还适用于不同的字体大小，因此它们20pt似乎相当稳定。如果有人知道它们对应的是什么，我很乐意相应地修改我的答案。

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

% Left-/right-aligned lines in gather, with spacing consistent with multline.
% Limitations: \tag{...}, if used, has to come BEFORE corresponding \gatherleft or \gatherright
\makeatletter
\newlength{\gatherlabelwidth}
\newcommand{\gatherbox}[2]{
    \settowidth{\gatherlabelwidth}{%
        \iftag@%
            \df@tag%
        \else%
            \stepcounter{equation}%
            \print@eqnum%
            \addtocounter{equation}{-1}%
        \fi%
    }
    \makebox[\displaywidth - \gatherlabelwidth - 20pt][#1]{$\displaystyle #2$}
}
\makeatother
\newcommand{\gatherleft}[1]{\gatherbox{l}{#1}}
\newcommand{\gatherright}[1]{\gatherbox{r}{#1}}

% for this MWE only:
\newcommand{\reset}{
    \setbox0=\vtop{\begin{multline}a\\b\\c\end{multline}}
    \setlength{\mytempheight}{\ht0}
    \addtolength{\mytempheight}{\dp0}
    \color{red}\vspace*{-\mytempheight}\vspace*{-0.5\baselineskip}}
\usepackage{color}
\newlength{\mytempheight}
\def\countervalue{0}
\newif\ifusetag

\begin{document}
    % Test some variations by uncommenting any line:
    % \tiny
    % \LARGE
    % \def\countervalue{6}
    % \def\countervalue{7}
    % \def\countervalue{8}
    % \def\countervalue{9}
    % \def\countervalue{96}
    % \def\countervalue{97}
    % \def\countervalue{98}
    % \def\countervalue{99}
    % \usetagtrue

    This is typeset with multline:
    \setcounter{equation}{\countervalue}\stepcounter{equation}\stepcounter{equation}
    \begin{multline}
        a\\b\\c
        \ifusetag
            \tag{my tag!}
        \else\fi
    \end{multline}

    \reset

    This is typeset with gather:
    \setcounter{equation}{\countervalue}
    \begin{gather}
        \gatherleft{a} \\
        b \\
        \ifusetag
            \tag{my tag!}
        \else\fi
        \gatherright{c}
    \end{gather}
\end{document}