Dash after theorem number: e.g. Theorem 1'

Question

I would like to present a second version of a theorem (of, say, Theorem 1) and I would like to denote it by Theorem 1'. I reckon the best way to go about it might be to define a new theorem style or environment? The thing is that I want this new theorem environment to be identical to the default one, except for the added dash after the theorem number. Moreover, if I attach a \label to the theorem, I would like the dash to appear as part of the theorem number when referring to it via the \refcommand. I don't know unfortunately how to go about this, in part because I don't know the default settings of the standard theorem environment, e.g. spacing, etc.

As an example, I would like \begin{dashedtheorem}[theorem title] TEXT... \end{dashedtheorem} to translate into Theorem 1' (theorem title). TEXT...

I realize that this question may have already been asked, e.g. New Theorem Style, but I could not find an answer that was enough to help me out.

I would be very grateful for any help with this!

EDIT: See also New theorem environment with manual theorem number.

Answer 1

Use the manualtheorem I gave you in another answer. There is no need that the primed theorem immediately follows the original one; it could also come before it, because we use the \label-\ref mechanism.

\documentclass{article}
\usepackage{amsthm}

\newtheorem{theorem}{Theorem}

\newtheorem{manualtheoreminner}{Theorem}
\newenvironment{manualtheorem}[1]{%
  \renewcommand\themanualtheoreminner{#1}%
  \manualtheoreminner
}{\endmanualtheoreminner}

\begin{document}

\begin{theorem}\label{foo}
This is a theorem.
\end{theorem}

\begin{manualtheorem}{\ref{foo}'}\label{baz}
This is a theorem.
\end{manualtheorem}

Here is \ref{foo} and \ref{baz}.

\end{document}

If you have hyperref loaded, you should use

\begin{manualtheorem}{\ref*{foo}}

in order not to make a wrong hyperlink.

Answer 2

We only need create a new counter and modify it so that it always gets printed with a dash after the number.

\documentclass{article}
\usepackage{amsthm}

% New counter that is printed like 1'
\newcounter{countD}
\renewcommand{\thecountD}{\arabic{countD}'}

\newtheorem{theorem}{Theorem}
\newtheorem{theoremD}[countD]{Theorem}

\begin{document}

    \begin{theoremD}[\textbackslash o/]
        lorem ipsum
        \label{theoD}
    \end{theoremD}

%   Set the number of the following theorem to the value 7, if you want to:
    \setcounter{countD}{7}\addtocounter{countD}{-1}
    \begin{theoremD}[\textbackslash o/]
        lorem ipsum
        \label{theoDD}
    \end{theoremD}

    \begin{theorem}[\textbackslash o/]
        lorem ipsum
        \label{theo}
    \end{theorem}

    We have the theorems \ref{theoD}, \ref{theoDD} and \ref{theo}.

\end{document}