Using ntheorem's counter inside theorem statement

Question

I'm using ntheorem package and I would like to use the theorem's counter within the theorem's statement. This is useful when you're defining a special object you wish to reference elsewhere, like:

\begin{theorem}
     For every foo, there exists a bar_\thcounter such that blah. 
\end{theorem}

Since the text has more than one bar defined, I wan't this particular one to be "bar_{1.1.2}" if the theorem is named as "Theorem 1.1.2".

Answer 1

The counter associated to a structure defined with \newtheorem{<env-name>}{<doc-name>} is named <env-name>, so you can use \the<env-name> (for the string including possible prefixes if the counter has been subordinated to another counter) or \number\value{<env-name>} (for just the value of the counter):

\documentclass{book}
\usepackage{ntheorem}

\newtheorem{theorem}{Theorem}[section]

\begin{document}

\chapter{Test Chapter}
\section{Test Section}
\begin{theorem}
     For every foo, there exists a bar\textunderscore\thetheorem\ such that blah. 
\end{theorem}

\end{document}

Another option, would be to use the \label, \ref mechanism:

\documentclass{book}
\usepackage{ntheorem}

\newtheorem{theorem}{Theorem}[section]

\begin{document}

\chapter{Test Chapter}
\section{Test Section}
\begin{theorem}\label{thm:test}
     For every foo, there exists a bar\textunderscore\ref{thm:test} such that blah. 
\end{theorem}

\end{document}

Notice the use of \textunderscore instead of simply _ which outside math mode will produce an error.