multline environment with multiple equation numbers

Question

I would like a and b to have an equation number of their own, while maintaining the overall layout that multline offers. Is that possible?

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

By default, all three lines have an equation number. What do you mean, exactly? — Bernard, Jul 15 '16 at 18:47
@Bernard I believe this is not true for multline. The manual says "Like equation, multline has only a single equation number". — bers, Jul 15 '16 at 18:49
what you probably want in a situation like this is gather. — barbara beeton, Jul 15 '16 at 18:51
@barbarabeeton Yes, "like" gather, "while maintaining the overall layout that multline offers". — bers, Jul 15 '16 at 18:52
Something like \begin{gather}\shoveleft ... \shoveright\end{gather}, if that worked. — bers, Jul 15 '16 at 18:58
the \shoveleft and \shoveright commands pfovided by mathtools only work with multlined (which is the "internal" equivalent of multline'), so they're no help. the only suggestion i can make offhand, then, is to add large\hspace*s to adjust the positioning of individual lines. requesting extending the applicability of the\shove...` commands seems not unreasonable. — barbara beeton, Jul 15 '16 at 18:59

score 2 · Answer 1 · answered Jul 15 '16 at 19:22

This maybe doesn't all the checks as in multline, but might be sufficient for your purposes:

\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}

Thanks, this looks exactly like what I was looking for. Spacing is not perfectly identical, though - nmultline contents seems to be set a little farther away from both left and right page limits. Probably due to some space around the \makeboxes, because I could not fix it by removing all white space from your MWE. — bers, Jul 17 '16 at 23:10
Or maybe 4em used in \displaywidth-4em is just an imperfect approximation. 20pt looks pretty good to me (or not, that messes with the equation numbers for the second line). — bers, Jul 17 '16 at 23:16

score 1 · Accepted Answer · answered Jul 18 '16 at 01:10

Based on @egreg's answer, this is what I came up with. It basically defines \gatherleft and \gatherright commands that are consistent in spacing with \multline, even with varying lengths of equation numbers (including tags).

It also works with different font sizes, so the 20pt seem to be pretty constant. If anyone knows what those correspond to, I would be happy to modify my answer accordingly.

\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}

multline environment with multiple equation numbers

2 Answers2

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