How do I make a Theorem n followed by a Theorem n' ?

Question

I am writing a paper where I have a stated a theorem, and then a short time later I state another theorem which is equivalent to the first. Is it possible to number them so that the second is a primed version of the first? In other words, if the first gets numbered as Theorem 7, I would like its equivalent version to be numbered as Theorem 7'. Ideas? Thanks.

Answer 1

\documentclass[a4paper]{article}

\newtheorem{thm}{Theorem}
\newenvironment{thmbis}[1]
  {\renewcommand{\thethm}{\ref{#1}$'$}%
   \addtocounter{thm}{-1}%
   \begin{thm}}
  {\end{thm}}

\begin{document}
\begin{thm}
$1+1=2$
\end{thm}
\begin{thm}\label{comm}
$a+b=b+a$
\end{thm}
\begin{thmbis}{comm}
$x+y=y+x$
\end{thmbis}
\begin{thm}
$0\ne0$
\end{thm}
\end{document}

In this way the "primed" theorem doesn't need to be near the main one. The thmbis environment takes as argument the label given to the main one.

If the "primed" theorem is always immediately after the main one, you can say

\newenvironment{thmbis}
  {\addtocounter{thm}{-1}%
   \renewcommand{\thethm}{\arabic{thm}$'$}%
   \begin{thm}}
  {\end{thm}}

If hyperref is used, one has to hack its automatic mechanism for creating hyperlinks; the following seems to work:

\makeatletter
\newcommand{\neutralize}[1]{\expandafter\let\csname c@#1\endcsname\count@}
\makeatother

\newtheorem{thm}{Theorem}

\newenvironment{thmbis}[1]
  {\renewcommand{\thethm}{\ref{#1}$'$}%
   \neutralize{thm}\phantomsection
   \begin{thm}}
  {\end{thm}}

Use \ref*{#1} if you don't want to make a link pointing to the "main theorem".

Answer 2

An idea but ... finally I undelete

\documentclass[12pt]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage{amssymb,amsmath,latexsym}

\newtheorem{theo}{Theorem}
\newcounter{biscompt}

\newtheorem{bis}[biscompt]{Theorem}
\renewcommand{\thebiscompt}{% 
\setcounter{biscompt}{\thetheo} 
\arabic{theo}'}
\usepackage{hyperref}   

\begin{document}

\chapter{}

\section{}  
\begin{theo}
On vérifie que :  $1\not= 1$
\end{theo}

\newpage
\section{}
\begin{theo}
On vérifie que :  $2\not= 3$
\end{theo}

\newpage   
\section{}   



\begin{theo}
On vérifie que :  $1\not= 1$
\end{theo}

\begin{bis}\label{bis}
On vérifie que :  $1\not= 1$
\end{bis}

\newpage
\section{}

With \ref{bis} 
\end{document}