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Suppose I have a tagged equation defined in an align environment, labelled using command \label.

Now I write another equation, but I want it tagged with the same number as the first equation plus a bespoke suffix, e.g. if the first equation is tagged (12), I want the second one tagged (12-2) or (12-b). I also want the suffixed tag to display as such if I reference the equation.

How can I achieve this?

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You may achieve your objective by using \tag -- a macro provide by the amsmath package, i.e., the same package that provides the align environment -- to override the default appearance of the "number" associated with an equation. The argument of \tag may contain a \ref statement that points to another equation.

enter image description here

\documentclass{article}
\usepackage{amsmath} % for "\tag" and "\eqref" macros and "align" env.
\setlength{\textwidth}{3in} % just for this example

\begin{document}
\setcounter{equation}{11}   % just for this example

\begin{align} 
a^2+b^2&=c^2 \label{pyth} \\
0+0&=0 \tag{\ref{pyth}-2} \label{triv} 
\end{align}
A cross-reference to equation \eqref{triv}.
\end{document}
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  • Thank you this works. It has a minor drawback though: the suffixed equation tag appears as an hyperref to the first equation. Is there a way to override that behavior? Commented Jul 3, 2021 at 13:59
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    @DaneelOlivaw - Just change \tag{\ref{pyth}-2} to \tag{\ref*{pyth}-2}. Note: \ref* is a command provided by the hyperref package, so it won't work unless the hyperref package is loaded.
    – Mico
    Commented Jul 3, 2021 at 14:03
  • Worked like a charm, thank you! Commented Jul 3, 2021 at 14:19

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