Numbering a set of horizontally distributed equations

Question

Having different horizontally distributed equations inside an align environment, only lines are numbered, but not individual equations.

How can the left equations be numbered using subequations? See also the question side-by-side equations, with equation numbers for each

\documentclass{article}
\usepackage{amsmath}
\begin{document}
  \begin{subequations}
    \begin{align} 
      %\label{a}
        a &= a ,&
      \label{b}
        b &= b ,\\
      %\label{c}
        c &= c ,&
      \label{d}
      d &= d .
    \end{align}
  \end{subequations}
\end{document}

The solution should be able to either:

• attach a label to the left equations, or
• modify the labels on the right to read (1a,b) and (1c,d)

I couldn't find anything in the documentation of amsmath or other packages.

Answer 1

You can define a custom nbytwosubequations environment to number subequations two by two, line by line.

Caution: this solution assumes that each line contains two subequations; the numbering will be wrong if one line contains only one subequation.

\documentclass{article}

\usepackage{amsmath}

\makeatletter
\newenvironment{nbytwosubequations}{%
  \refstepcounter{equation}%
  \protected@edef\theparentequation{\theequation}%
  \setcounter{parentequation}{\value{equation}}%
  \setcounter{equation}{0}%
  \def\theequation{%
    \theparentequation\alph{equation}%
    \addtocounter{equation}{1},\alph{equation}%
  }%
  \ignorespaces
}{%
  \setcounter{equation}{\value{parentequation}}%
  \ignorespacesafterend
}
\makeatother

\begin{document}

  \begin{nbytwosubequations}
    \begin{align} 
      a &= a,  &
      b &= b   \label{ab} \\
      c &= c,  &
      d &= d. \label{cd}
    \end{align}
  \end{nbytwosubequations}

\end{document}

Answer 2

You can use minipages:

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\noindent\begin{subequations}
\begin{minipage}{.5\textwidth}
    \begin{align} 
      \label{a}
        a &= a, \\
      \label{c}
        c &= c ,
    \end{align}
\end{minipage}%
\begin{minipage}{.5\textwidth}
    \begin{align} 
      \label{b}
        b &= b ,\\
      \label{d}
      d &= d .
    \end{align}
\end{minipage}
\end{subequations}

\end{document}

Here's another option, adapted from an example from mathmode; numbering now follows the desired sequence:

\documentclass{article}
\usepackage{amsmath}

\newcounter{mySubCounter}
\newcommand{\foureqn}[8]{%
\setcounter{mySubCounter}{0}
\let\OldTheEquation\theequation%
\renewcommand{\theequation}{\OldTheEquation\alph{mySubCounter}}%
\noindent%
\begin{minipage}{.5\textwidth}
\begin{align}
\refstepcounter{mySubCounter}
#1 &= #2 \label{sub\theequation}\\
\addtocounter{equation}{-1}
\addtocounter{mySubCounter}{2}
 #5 &= #6 \label{sub\theequation}
\end{align}
\end{minipage}%
\addtocounter{equation}{-1}%
\addtocounter{mySubCounter}{-1}%
\begin{minipage}{.49\textwidth}
\begin{align}
#3 &= #4 \label{sub\theequation} \\
\addtocounter{equation}{-1} 
\addtocounter{mySubCounter}{2}
#7 &= #8 \label{sub\theequation}
\end{align}
\end{minipage}\par\medskip%
\let\theequation\OldTheEquation}

\begin{document}

Some references: \eqref{sub1a}, \eqref{sub1b}, \eqref{sub1c}, and~\eqref{sub1d}

\foureqn{a}{a+b,}{b+c}{b,}{d+c+e}{c,}{d}{d.}

Some other references: \eqref{sub2a}, \eqref{sub2b}, \eqref{sub2c}, and~\eqref{sub2d}

\foureqn{p+q}{r,}{s+t+u}{v+w,}{x}{y+z,}{z}{z.}

\end{document}

Each subequation has an automatic assigned label for cross-referencing; the label is of the form sub<number>, where <number> is the string used to number the subequation.

Answer 3

Not straightforward input, but it seems to work; in the image I added a vertical rule to show the middle of the page. Not perfect, either, but in my opinion this is not so good an idea.

\documentclass{article}
\usepackage{amsmath,lipsum}

\makeatletter
\newcommand{\leftlabel}[1]{&&
  \refstepcounter{equation}\ltx@label{#1}%
  \tagform@{\theequation}&&}
\makeatletter

\begin{document}
\lipsum[2]
\begin{subequations}
\begin{flalign}
&&a &= a,\leftlabel{a} & b &= b \label{b} &&\\
&&c &= c,\leftlabel{c} & d &= d \label{d} &&
\end{flalign}
\end{subequations}

\end{document}