Refer to an equation with multiple labels

Question

I have the following equation:
begin{equation}tag{E$_{n}$}label{E}
 f_n(x)=a
end{equation}

When I write eqref{E} I am referred to the equation and it is written E_n
My question is: how to be referred to the same equation and write for example E_{n-1} rather then E_n?

egreg · Accepted Answer

You can do like this:
documentclass{article}
usepackage{amsmath}
usepackage{hyperref}

DeclareRobustCommand{tagindex}{ensuremath{_{defaulttagindex}}}
newcommand{defaulttagindex}{n}
DeclareRobustCommand{indexedref}[2][n]{%
  begingroupdefdefaulttagindex{#1}ref{#2}endgroup
}
DeclareRobustCommand{indexedeqref}[2][n]{%
  begingroupdefdefaulttagindex{#1}eqref{#2}endgroup
}

begin{document}

begin{equation}tag{Etagindex}label{E}
 f_n(x)=a
end{equation}

We refer to~indexedeqref{E}, but also to~indexedeqref[n-1]{E}.

end{document}

Marijn · Answer

For this use case you can use the command hyperref[label]{text} from the hyperref package. It creates a (clickable) link to the label with the text being shown in the document (and not the default label text).
MWE:
documentclass{article}
usepackage[colorlinks]{hyperref}
usepackage{amsmath}
begin{document}
begin{equation}tag{E$_{n}$}label{E}
 f_n(x)=a
end{equation}
See Equation eqref{E}, also known as Equation (hyperref[E]{E$_{n-1}$}).
end{document}

Result:

Refer to an equation with multiple labels

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