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I am drafting a document that has an important system of equations, which I would like to highlight by using Roman numerals to enumerate them. I thought about using the subequations environment, but I do not know how to actually change the enumeration and style for only a specific system of equations. Also, afterwards, I would like to derive a new system from the current one, and I would like to enumerate the new one with Roman numerals with stars (asterisks) to the right of the Roman numerals. I was unable to find the answer via web search. I thank all helpers for any assistance.

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  • 1
    Should the equation numbers increment sequentially all through the document? Please advise.
    – Mico
    Jun 26, 2022 at 6:47
  • @Mico no, I want the numerals to be I, II, III... regardless of the numbering of the other equations in the document
    – user209867
    Jun 26, 2022 at 6:51

2 Answers 2

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(I've updated the answer to handle the possibility -- not mentioned by the OP, but likely of some relevance to at least some readers -- that the hyperref package is loaded and that correct hyperlinking from cross-references to equations and the corresponding equations is required.)

In the following solution, I assume that the hyperref package is loaded. If this assumption is incorrect, you should remove all instances of \renewcommand\theHequation{...}.

The solution pursued here creates Roman-numeral and Roman-numeral-plus-star equation "numbers" by (a) resetting the equation counter to 0 (or whatever value is deemed appropriate), (b) redefining the \theequation macro as needed, and (c) redefining the \theHequation macro. The third step isn't needed if your document doesn't load the hyperref package. In addition, the solution defines the utility macros \SaveEqCtr and \RestoreEqCtr to allow toggling back and forth between ordinary and special forms of the equation number.

enter image description here

\documentclass{article}

\usepackage{amsmath}  % for gather environment
\usepackage[colorlinks,allcolors=blue]{hyperref} % optional
\usepackage{cleveref} % for \cref macro

% Create a new counter, to enable toggling back and forth 
%   between ordinary and special forms of equation counters
\newcounter{SaveEqnCntr}
\newcommand\SaveEqCtr{\setcounter{SaveEqnCntr}{\value{equation}}}
\newcommand\RestoreEqCtr{\setcounter{equation}{\value{SaveEqnCntr}}}

\begin{document}

\begin{equation} 
     0+0=0 \label{a} 
\end{equation}
\SaveEqCtr  % Store the current value of 'equation' counter (=1)

\begingroup % Limit the scope of the next few directives to the current TeX group
\setcounter{equation}{0}
%% Display equation counter using Roman numerals:
\renewcommand\theequation{\Roman{equation}} 
%% If you use 'hyperref', choose a unique identifier, e.g., 
%%   the string 'AA', to redefine the '\theHequation' macro
\renewcommand\theHequation{AA\arabic{equation}} 
\begin{gather} 
     1+1=2 \label{b}  \\ 
     1+2=3 \label{c}  \\
     2+1=3 \label{d}
\end{gather}
\endgroup

\RestoreEqCtr % restore 'standard' form of equation counter
\begin{equation} 
     2+2=4 \label{e} 
\end{equation}
\SaveEqCtr    % Store the current value of 'equation' counter (=2)

\begingroup % Limit the scope of the next few directives to the current TeX group
\setcounter{equation}{0}
%% Display equation counter using Roman numerals plus a 'star'
\renewcommand\theequation{\Roman{equation}$^{\star}$}
%% If you use 'hyperref', choose a unique identifier, e.g., 
%%   the string 'BB', to redefine the '\theHequation' macro
\renewcommand\theHequation{BB\arabic{equation}}
\begin{gather} 
     3+3=6 \label{f} \\ 
     3+4=7 \label{g} \\
     4+3=7 \label{h}
\end{gather}
\endgroup

\RestoreEqCtr % restore 'standard' form of equation counter
\begin{equation} 
     4+4=8 \label{i} 
\end{equation}

\noindent
Cross-references to \cref{b,c,d}, \cref{f,g,h}, and \cref{a,,e,,i}.
\end{document}
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  • thank you very much
    – user209867
    Jun 26, 2022 at 7:24
  • 1
    @kroner - You're most welcome. Please note that I've augmented my answer to allow for the possibility that the document employs the hyperref package and hence that the hyperlinks between the cross-referencing call-outs and the associated equations have to "work" as expected.
    – Mico
    Jun 26, 2022 at 9:51
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I would use \tag. After all, you just have one special group of equations and you'll need a label for each one of them.

Using (modified) Mico's code

\documentclass{article}
\usepackage{amsmath}  % for gather environment
\usepackage{cleveref} % for \cref macro

\newcounter{maineq}
\renewcommand{\themaineq}{\Roman{maineq}}
\NewDocumentCommand{\maineq}{sm}{%
  \IfBooleanTF{#1}{% call is \maineq*
    \tag{\ref{#2}*}\label{#2*}%
  }{% no *
    \stepcounter{maineq}%
    \tag{\themaineq}\label{#2}%
  }%
}


\begin{document}

Cross-references to \cref{good,bad,ugly} and to
\cref{good*,bad*,ugly*}

\begin{equation} 0+0=0 \label{a} \end{equation}

\begin{gather} 
  1+1=2 \maineq{good} \\ 
  1+2=3 \maineq{bad} \\
  2+1=3 \maineq{ugly}
\end{gather}

\begin{equation} 2+2=4 \label{e} \end{equation}

\begin{gather} 
  3+3=6 \maineq*{good} \\ 
  3+4=7 \maineq*{bad} \\
  4+3=7 \maineq*{ugly}
\end{gather}

\begin{equation} 4+4=8 \label{i} \end{equation}

\end{document}

enter image description here

Using cleveref is not mandatory and cross references with \ref will works as well. Anyway, if you use hyperref, you may want to change \ref in the definition of \maineq into \ref*, precisely

\NewDocumentCommand{\maineq}{sm}{%
  \IfBooleanTF{#1}{% call is \maineq*
    \tag{\ref*{#2}*}\label{#2*}%
  }{% no *
    \stepcounter{maineq}%
    \tag{\themaineq}\label{#2}%
  }%
}

If you don't, the tags in the starred equations will be hyperlinks that point to the original equations.

As a bonus, if you use again \maineq*{good*}, you will get two asterisks.

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