How do I repeat a theorem number?

Question

Often in my papers I want to mention theorems in the introduction that are introduced in more detail and proved in a later section.

I'd like to include the statement of these theorems twice over, each time numbered as the theorem would naturally appear in its later section.

My current solution (using amsthm) is, e.g.

%(in the introduction)
\newtheorem*{thm:associativity}{Theorem \ref{thm:associativity}}
\begin{thm:associativity}
Lorem ipsum ...
\end{thm:associativity}

%(in a later section)
\begin{thm}
\label{thm:associativity}
Lorem ipsum ...
\end{thm}

Can anyone think of a cleaner solution?

Answer 1

If you want to allow yourself to vary the text between the two occurrences, here's an alternative:

\documentclass{article}

\usepackage{amsthm}

\makeatletter
\newtheorem*{rep@theorem}{\rep@title}
\newcommand{\newreptheorem}[2]{%
\newenvironment{rep#1}[1]{%
 \def\rep@title{#2 \ref{##1}}%
 \begin{rep@theorem}}%
 {\end{rep@theorem}}}
\makeatother


\newtheorem{theorem}{Theorem}
\newreptheorem{theorem}{Theorem}
\newtheorem{lemma}{Lemma}
\newreptheorem{lemma}{Lemma}

\begin{document}

\begin{reptheorem}{myAmazingTheorem}
That theorem again
\end{reptheorem}

\begin{theorem}
\label{myAmazingTheorem}
A theorem
\end{theorem}
\begin{lemma}
\label{anInsignificantLemma}
A lemma
\end{lemma}

\begin{replemma}{anInsignificantLemma}
That theorem again
\end{replemma}


\end{document}

Probably needs a little tweaking to suit different theorem styles and the like.

Answer 2

What about the thmtools bundle? Check out page 6 of the guide: there's a 'restatable' environment.

Answer 3

Here's a solution I like even though it's not very clever. You simply temporarily redefine \thetheorem (or in your case, \thethm):

% in the introduction
{
\renewcommand{\thetheorem}{\ref{thm:associativity}}
\begin{theorem}
  Lorem ipsum ...
\end{theorem}
\addtocounter{theorem}{-1}
} % note: these braces are here to take advantage of LaTeX scoping ... 
  % \thetheorem is returned to its rightful definition outside of this group

% elsewhere
\begin{theorem} \label{thm:associativity}
  Lorem ipsum ...
\end{theorem}

You can also define an environment on the same principle

% repeat theorems, with amsthm 
% args = type,counter,reference
\newenvironment{repthm}[3] 
{ 
  \bgroup 
  \addtocounter{#2}{-1} 
  \expandafter\def\csname the#2\endcsname{\ref{#3}} 
  \def\foo{\end{#1}} 
  \begin{#1} 
} 
{ 
  \foo 
  \egroup 
} 

Answer 4

Here is how I do it:

\documentclass{article}

\usepackage{amsthm}

    \theoremstyle{plain}
    \newtheorem{thm}{Theorem}[section]


    \newtheoremstyle{TheoremNum}
        {\topsep}{\topsep}              %%% space between body and thm
        {\itshape}                      %%% Thm body font
        {}                              %%% Indent amount (empty = no indent)
        {\bfseries}                     %%% Thm head font
        {.}                             %%% Punctuation after thm head
        { }                             %%% Space after thm head
        {\thmname{#1}\thmnote{ \bfseries #3}}%%% Thm head spec
    \theoremstyle{TheoremNum}
    \newtheorem{thmn}{Theorem}


\begin{document}

\begin{thmn}[\ref{T:1}] $1+1 = 2$
\end{thmn}

\begin{thm}\label{T:1} $1+1 = 2$
\end{thm}

\end{document}

Answer 5

A somewhat "sketchy" solution: there's a package by Ulrich Diez called theomac.sty that's floating online but isn't on CTAN, and which I've used once before. The latest version I'm aware of contains the string "2007/06/18 v1.00beta4 Define macros from theorems" in it, and can be found, say, in the discussions here or here (or an older version on Google Groups where you can download it). You use it somewhat as follows:

\usepackage{theomac} %To repeat theorems
\newtheoremWithMacro{rtheorem}[theorem]{Theorem}

...

\begin{rtheorem}[\foo]
  $1 + 1 = 2$ 
\end{rtheorem}

(Or any name instead of rtheorem.) Then you can write \foo anywhere you want the theorem to appear, without having to repeat the body.

Answer 6

Following is how I implement Andrew Stacey's suggestion. The difference is that I use ntheorem to simulate amsthm style; and I'm using \autoref to have a single restatement environment which works for theorems, corollaries, anything - based on just the label. It's not elegant or terse, but it should be useful if you use ntheorem. Of course, if you don't care about AMS styles, just tweak the \newtheoremstyle commands to your liking (while keeping the styles for the statement and the restatement in synch).

Also, the code allows for restatement both with and without additional text after the theorem number.

\documentclass{article}

\usepackage{hyperref}
\usepackage[hyperref]{ntheorem}

\makeatletter
\def\ifempty#1{%
 \def\@tmp@a{#1}%
 \ifx\@tmp@a\@empty%
}

\newtheoremstyle{ams-theorem}%
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2\theorem@separator]}%
  {\item[\hskip\labelsep {\theorem@headerfont ##1\ ##2}{\normalfont\ (##3)}{\theorem@headerfont
  \theorem@separator}]}
% The following is a dummy style, we only need it so ntheorem uses its
% unnumbered version properly
\newtheoremstyle{ams-restatedtheorem}
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2\theorem@separator]}%
  {\item[\hskip\labelsep {\theorem@headerfont ##1\ ##2}{\normalfont\ (##3)}{\theorem@headerfont
  \theorem@separator}]}
% ... this is the one that's actually used, and in fact, we only use the
% 3-argument variant
\newtheoremstyle{nonumberams-restatedtheorem}%
  {\item[\theorem@headerfont \hskip\labelsep ##1\theorem@separator]}%
  {\item[\hskip\labelsep \theorem@headerfont ##3\theorem@separator]}%

\theoremstyle{ams-theorem}
\theoremheaderfont{\normalfont\bfseries}
\theorembodyfont{\itshape}
\theoremseparator{.}
\theoremindent0cm
\theoremnumbering{arabic}
\theoremsymbol{}

\newtheorem{theorem}{Theorem}
% and here you would have environments such as lemma, corollary, etc.
% (possibly with \newaliascnt commands to get them to use the same
% counter)


\theoremstyle{ams-restatedtheorem}
% the argument of restatedtheorem* shouldn't ever actually be used
\newtheorem*{restatedtheorem*}{}
\newenvironment{restatement}[2][]{%
 \ifempty{#1}
  \begin{restatedtheorem*}[\autoref*{#2}]%
 \else%
  \begin{restatedtheorem*}[\autoref*{#2} (#1)]%
 \fi%
}%
{\end{restatedtheorem*}}
\makeatother

\begin{document}
\begin{theorem}
\label{myfirst}
This is my first theorem.
\end{theorem}
\begin{theorem}
\label{mysecond}
This is my second theorem.
\end{theorem}
\begin{restatement}{myfirst}
This is a restatement of my first theorem.
\end{restatement}
\begin{restatement}[fancy version]{mysecond}
Hear ye, hear ye, the most excellent seconde theoreme!
\end{restatement}
\end{document} 

Answer 7

Finally wanted to do this myself and decided that I didn't like my solution. Here's an alternative. It works for amsthm. Don't know how it would work with other theorem-builders.

\ExplSyntaxOn
\DeclareDocumentEnvironment{repetition}{m o}
{
  \tl_set_eq:Nc \l_tmpa_tl {#1}
  \IfNoValueTF{#2}{
    \tl_set:Nx \l_tmpb_tl {
      \tl_item:Nn \l_tmpa_tl {1}
      {\tl_item:Nn \l_tmpa_tl {2} }
      {}
      {\tl_item:Nn \l_tmpa_tl {4}}
    }
  }
  {
    \tl_set:Nx \l_tmpb_tl {
      \tl_item:Nn \l_tmpa_tl {1}
      {\tl_item:Nn \l_tmpa_tl {2} }
      {}
      {\tl_item:Nn \l_tmpa_tl {4} ~ \exp_not:N \ref{#2} }
    }
  }
  \tl_use:N \l_tmpb_tl
}
{
  \use:c {end #1}
}
\ExplSyntaxOff

This defines a single new environment repetition. The environment takes two arguments, a first mandatory one which is the name of the environment to be repeated, and a second optional one which is the label of the environment to be repeated.

The way it works is as follows. When, say, \begin{proposition} is invoked then eventually the command \proposition is executed. This is a token list consisting of the basic theorem-builder and three arguments which set up how that basic theorem-builder will behave. The first sets the style, so we want to keep that. The second says which counter to use, so we want to throw that away. The third says what the title text should be, so we want to modify that slightly if we're given a label. The L3 stuff does that modification before invoking the token list and thus starting the theorem-builder on its merry way.

Answer 8

Another simple solution that works in simple circumstances:

After the other theorem environments are defined,

\theoremstyle{empty}
\newtheorem{duplicate}{NameIgnored}

Then in the body of the document

\begin{theorem}[My theorem tile]
\label{thm:worth.repeating}
An insightful truth.
\end{theorem}

and elsewhere

\begin{duplicate}[Theorem~\ref{thm:worth.repeating} (My theorem title)]
An insightful truth (again).
\end{duplicate}

Answer 9

Here's an answer, maybe not that fancy like others but it just works.

Use your favorite theorem enviroment and then replicate that enviroment attaching the reference.

For example:

... 
\begin{document}

{\bf Theorem \ref{theo1}.} {\it This is my theorem} 


\begin{theorem}\label{theo1}
This is my theorem
\end{theorem}

\end{document}