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}

Answer 1

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}

Answer 2

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}