Typesetting a slight variation to a theorem by adding a * after the numbering of the theorem

Question

Suppose I have a theorem with numbers and everything like this:

Theorem 1.2.3: Blah Blah Blah...

Sometimes you want to do a slight variation to the same theorem, or write it with a different language, etc. Mathematicians use a notation like this then:

Theorem 1.2.3*: A different Blah Blah Blah

So at the end the text looks like:

Theorem 1.2.3: Blah Blah Blah...

Theorem 1.2.3*: A different Blah Blah Blah

How do you do the * thing in LaTeX?

EDIT My code for the non-starred version looks like this:

\documentclass[11pt]{article}
\usepackage{amsmath} 
\usepackage{amsthm}
\newtheorem{thm}{Theorem}[subsection]
\begin{document}
\section{Blah}
\subsection{Blah Blah}


Typesetted as Theorem 1.1.1:
\begin{thm}[Theorem 1]
Blah Blah Blah
\end{thm}


Typesetted as Theorem 1.1.2
\begin{thm}[Theorem 2]
Blah Blah Blah
\end{thm}


The first theorem with slight variation:
I want this to be typesetted as Theorem 1.1.1*: Blah Blah Blah
%% The code for this??? %%
\end{document}

Answer 1

You can declare a new theorem and just make its counter look like the regular counter with the addition of a *. If you want to refer to a theorem that is somewhere else in the document, use the \label-\ref system and supply the new number via \thmstarnum{\ref{<label>}}:

\documentclass{article}
\usepackage{amsthm}
\newtheorem{thm}{Theorem}[subsection]
\newtheorem{thm*}{Theorem}% This is a fake theorem environment
% update the counter to look the same as that of 'thm' + '*'

\makeatletter
\expandafter\newcommand\csname thethm*default\endcsname{\thethm*}
\newcommand{\thmstarnum}[1]{\expandafter\gdef\csname thethm*\endcsname{#1*}}
\thmstarnum{\thethm}
\expandafter\g@addto@macro\csname endthm*\endcsname{\thmstarnum{\thethm}}
\makeatother

\begin{document}

\section{This is a section}
\subsection{This is a subsection}
Here is some text.
\begin{thm}\label{thm:first}
This is the first theorem.
\end{thm}
Here is some more text, and then another theorem.
\begin{thm}[Something]
This is the second theorem.
\end{thm}
Now we look at the same theorem again.
\begin{thm*}[Something else]
This is the second theorem again.
\end{thm*}
Now let's review the first theorem again.
\thmstarnum{\ref{thm:first}}
\begin{thm*}
This is the first theorem again.
\end{thm*}
Now the rest of the document with some more theorems.
\begin{thm}
This is the third theorem.
\end{thm}
Let's revisit the third theorem.
\begin{thm*}
This is the third theorem again.
\end{thm*}
The conclusion.

\end{document}

Answer 2

If you want to allow that the “variant theorem” appears anywhere (maybe even before the standard one), you need to use \label and \ref:

\documentclass[11pt]{article}
\usepackage{amsmath} 
\usepackage{amsthm}

\newtheorem{thm}{Theorem}[subsection]
\newtheorem*{varthm+inner}{\varthmname}
\newcommand{\varthmname}{}
\newenvironment{varthm}[2][Theorem]
 {\renewcommand{\varthmname}{#1 \ref{#2}*}\begin{varthm+inner}}
 {\end{varthm+inner}}

\begin{document}

\section{Blah}
\subsection{Blah Blah}

Typeset as Theorem 1.1.1:
\begin{thm}\label{test}
Blah Blah Blah
\end{thm}


Typeset as Theorem 1.1.2
\begin{thm}
Blah Blah Blah
\end{thm}

Typeset as Theorem 1.1.1*
\begin{varthm}{test}
Blah blah
\end{varthm}

\end{document}

I added an optional argument, so you can typeset also variant lemmas or whatnot, by calling

\begin{varthm}[Lemma]{label}

The attribution argument is honored, by calling

\begin{varthm}{label}[Somebody]

Example code

Typeset as Theorem 1.1.1:
\begin{thm}[A. Uthor]\label{test}
Blah Blah Blah
\end{thm}

Typeset as Theorem 1.1.2
\begin{thm}
Blah Blah Blah
\end{thm}

Typeset as Theorem 1.1.1*
\begin{varthm}{test}[W. Riter]
Blah blah
\end{varthm}