Cfrgtkky

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).

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{theequationensuremath{#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}

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}

patchcmdsubequations

  {deftheequation{theparentequationalph{equation}}}

  {deftheequation{theparentequationapost{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}

An alternative is the modified answer from here:

documentclass{article}

usepackage{amsmath}

makeatletter

newcommand*ifcounter[1]{%

  ifcsname c@#1endcsname%

    expandafter@firstoftwo%

  else%

    expandafter@secondoftwo%

  fi%

}%

makeatother







newcommandEqFamTag[2][roman]{%

ifcounter{#2}{%

expandafteraddtocounter{#2}{1}%

expandafteraddtocounter{#2Prev}{1}

xdeftemp{csname #2 Eqendcsname ifnumvalue{#2Prev}>0 space(csname #1endcsname{#2Prev})fi}%

globalexpandafterletcsname #2arabic{#2}endcsnametemp%

tag{temp}%

}{%

globalexpandafternewcounter{#2}%

globalexpandafternewcounter{#2Prev}

expandafteraddtocounter{#2}{1}%

xdeftemp{theequation ifnumvalue{#2Prev}>0 space(csname #1endcsname{#2Prev})fi}%

xdefeqonfamily{theequation}%

globalexpandafterletcsname #2 Eqendcsnameeqonfamily%

globalexpandafterletcsname #2arabic{#2}endcsnametemp%

tag{temp}%

expandafteraddtocounter{equation}{1}

}%

}%



newcommandmyFamRef[2][1]{csname #2#1endcsname}



begin{document}

section{test}

begin{equation}

f(x)=3cdot x

end{equation}



begin{equation}

    x=4cdot y EqFamTag{MyFamily}

end{equation}



As an example:



begin{equation}

    x=4cdot 2=8EqFamTag{MyFamily}

end{equation}



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



begin{equation}

    x=4cdot y 

end{equation}



Another example of :



begin{equation}

    x=4cdot 2=8EqFamTag{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:

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} \

  5label{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}