Customize subequation numbering

Question

I need to customize equation numbering as follows:

Some text ...

    a+b  (5)
    3+2  (5')

... some more text....

    c+d  (6)

The idea is that (5') is an example of (5). I thought of using subequations, but I am not sure how to customize the inner numbering, as it would generate 5a and 5b, or 5.i, 5.ii, etc.... If I use \tag{} instead, I am not sure how to get the previous equation number (5, in the example).

Answer 1

You can define an \eqorn command. If you use \eqorn[''] you get two primes; any math symbol can be specified in the optional argument.

\documentclass{article}
\usepackage{amsmath}

\newcommand{\eqorn}[1][']{\tag{\theequation\ensuremath{#1}}}

\begin{document}

Some text ...
\begin{gather}% or align or whatnot
a+b \label{main} \\
3+2 \eqorn \label{secondary}
\end{gather}
... some more text....
\begin{equation}
c+d
\end{equation}
with references to \eqref{main} and \eqref{secondary}.

\end{document}

Answer 2

Here's an approach using expl3's \prg_replicate:nn to produce n copies of ' to number the equations in subequations and etoolbox's \patchcmd to insert that in the subequations environment.

\documentclass{article}
\usepackage{amsmath}
\usepackage{expl3}

% Creating a command to print a number of apostrophes
\ExplSyntaxOn
\cs_new:Npn \apost #1
  { \__andy_apost:cn { c@ #1 } }
\cs_new:Npn \__andy_apost:Nn #1
  {
    \ensuremath {
      \prg_replicate:nn { #1 - 1 } { {'} }
    }
  }
\cs_generate_variant:Nn \__andy_apost:Nn { c }
\ExplSyntaxOff

% Patching-in that command in \subequations
\usepackage{etoolbox}
\patchcmd\subequations
  {\def\theequation{\theparentequation\alph{equation}}}
  {\def\theequation{\theparentequation\apost{equation}}}
  {}{\FAIL}

\begin{document}

\begin{subequations}
\begin{equation}
  a+b
\end{equation}
\begin{equation}
  3+2
\end{equation}
\begin{equation}
  5
\end{equation}
\end{subequations}

\end{document}

Answer 3

An alternative is the modified answer from here:

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newcommand*\ifcounter[1]{%
  \ifcsname c@#1\endcsname%
    \expandafter\@firstoftwo%
  \else%
    \expandafter\@secondoftwo%
  \fi%
}%
\makeatother



\newcommand\EqFamTag[2][roman]{%
\ifcounter{#2}{%
\expandafter\addtocounter{#2}{1}%
\expandafter\addtocounter{#2Prev}{1}
\xdef\temp{\csname #2 Eq\endcsname \ifnum\value{#2Prev}>0 \space(\csname #1\endcsname{#2Prev})\fi}%
\global\expandafter\let\csname #2\arabic{#2}\endcsname\temp%
\tag{\temp}%
}{%
\global\expandafter\newcounter{#2}%
\global\expandafter\newcounter{#2Prev}
\expandafter\addtocounter{#2}{1}%
\xdef\temp{\theequation \ifnum\value{#2Prev}>0 \space(\csname #1\endcsname{#2Prev})\fi}%
\xdef\eqonfamily{\theequation}%
\global\expandafter\let\csname #2 Eq\endcsname\eqonfamily%
\global\expandafter\let\csname #2\arabic{#2}\endcsname\temp%
\tag{\temp}%
\expandafter\addtocounter{equation}{1}
}%
}%

\newcommand\myFamRef[2][1]{\csname #2#1\endcsname}

\begin{document}
\section{test}
\begin{equation}
f(x)=3\cdot x
\end{equation}

\begin{equation}
    x=4\cdot y \EqFamTag{MyFamily}
\end{equation}

As an example:

\begin{equation}
    x=4\cdot 2=8\EqFamTag{MyFamily}
\end{equation}

The equation: \myFamRef[2]{MyFamily} is the first example/member of the equation \myFamRef{MyFamily}

\begin{equation}
    x=4\cdot y 
\end{equation}

Another example of :

\begin{equation}
    x=4\cdot 2=8\EqFamTag{MyFamily}
\end{equation}

The equation: \myFamRef[3]{MyFamily} is the second example/member of the equation \myFamRef{MyFamily}

Another family of equations is:

\begin{equation}
    f(x)=3+y \EqFamTag{NewFam}
\end{equation}

and a member of it is:

\begin{equation}
    f(6)=9 \EqFamTag{NewFam}
\end{equation}

\end{document}

Output:

Answer 4

It is very easy to have a first (main) subequation numbered as an ordinary equation, the subequations standard numbering starting only at the second subequation:

\documentclass{article}
\usepackage{amsmath}
\setcounter{equation}{4}

\begin{document}
Some text . . . . . .
\begin{subequations}
\addtocounter{equation}{-1}
\begin{gather}
  a+b \label{main} \\
  3+2 \label{ex-1} \\
  5\label{ex-2}
\end{gather}
\end{subequations}.
. . . . . .Some more text . . . . . .
\begin{equation}
  c + d
\end{equation}

\eqref{ex-1} and \eqref{ex-2} illustrate \eqref{main}.

\end{document}