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}

enter image description here

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.

score 12 · Answer 1 · edited Jun 03 '17 at 16:37

12

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}

enter image description here

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}

enter image description here

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.

edited Jun 03 '17 at 16:37

Moriambar

11,466

answered May 06 '13 at 14:52

Gonzalo Medina

505,128

The numbering doesn't match. I think the OP wants the first two on the first line and the last two on the second line, which is a bit trickier. – jub0bs May 06 '13 at 14:59
Ok, haven't thought about minipage command. Nice solution. Still - as @Jubobs says - the numbering goes vertically first in your example instead of horizontally. I fear I can't simply modify your solution by using horizontal aligns, as then the vertical alignment is not maintained any more. Any ideas how to fix that?? – jjdb May 06 '13 at 15:03
@jjdb I added another option to my answer. – Gonzalo Medina May 06 '13 at 15:17
@jjdb new update. Alignment and numbering are now OK. – Gonzalo Medina May 06 '13 at 15:27
@GonzaloMedina Thanks for the update. I see the labels are inside the command definition. Can your solution be used various times inside an document, or is it needed to introduce a new command for each subequations? – jjdb May 07 '13 at 13:28
@jjdb with the new update you can use \foureqn as many times as you wish without introducing new commands. Now the labels will automatically change every time the command is invoked. Label for subequation (3a), for example, will be sub3a, so you can simply say \ref{sub3a}. – Gonzalo Medina May 07 '13 at 15:14
@GonzaloMedina thanks for the update. The example works nicely, and the result looks good. Still what worries me is the different syntax than using the usual align environment. – jjdb May 08 '13 at 15:59
@jjdb and what exactly worries you? Internally, the syntax used is precisely the one of an align environment. – Gonzalo Medina May 08 '13 at 16:58
@GonzaloMedina Now it is a command, before it was an environment. – jjdb May 09 '13 at 12:17

score 5 · Accepted Answer · edited Jun 12 '17 at 13:10

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.

enter image description here

\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}

score 4 · Answer 3 · edited Jun 09 '17 at 19:24

4

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}

enter image description here

edited Jun 09 '17 at 19:24

David Carlisle

757,742

answered May 06 '13 at 15:37

egreg

1,121,712

Just to clarify: what does " this" refer to in "this is not so good an idea"? Your solution or the OP's desired output? – jub0bs May 06 '13 at 15:42
@Jubobs The OP's desired output: associating an equation to its number is not at all immediate. – egreg May 06 '13 at 15:45
Yes, it's more difficult to parse than four subequations, each on its own line. – jub0bs May 06 '13 at 15:46
I agree on that. I actually favor the second requested solution, which has also been answered by @Jubobs – jjdb May 06 '13 at 16:21
@jjdb Actually, I don't like the output of my solution; numbers are too close to the right-hand-side subequations, which can be confusing. I think you'd be better off dedicating one line to each subequation... – jub0bs May 06 '13 at 19:04
@Jubobs It looks to me as it is the same spacing as in the plain align environment as in the example I've provided. Already there it might be confusing (Are both equations or only te right ones numbered?). When one has more material in each equation, the asymmetric spacing should not matter that much anymore, I guess. – jjdb May 06 '13 at 20:36
1

@jjdb Yes, it may not be problematic all the time; it's a case-by-case thing. My tip would just be: avoid any opportunity to confuse your readers :) – jub0bs May 06 '13 at 21:17

Numbering a set of horizontally distributed equations

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