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.