How to mark a theorem not under its section?

Question

Take an example, if a theorem written in section 1, it will be marked Theorem 1.x . In spite of those, can it be marked Theorem 2.x without moved into section 2?

It sounds a bit strange, but it is a problem someone will really meet once in a while, such as in using Laeqed to convert a single theorem to an image file. In this case it will be shown as Theorem 0.1 since the information of section is not concluded.

Answer 1

You can redefine the representation of the counter associated to the environment that gives you the theorem:

\documentclass{article}
\usepackage{amsthm}

\newtheorem{theo}{Theorem}[section]
\begin{document}
\section{Test Section One}

{
\renewcommand\thetheo{2.\arabic{theo}}
\begin{theo}
test
\end{theo}
}

\end{document}

The pair of outer braces is just to keep the redefinition local.

Using the \label, \ref mechanism, the section number doesn't have to bee hard-coded:

\documentclass{article}
\usepackage{amsthm}

\newtheorem{theo}{Theorem}[section]
\begin{document}

\section{Test Section One}
\label{sec:one}
{
\renewcommand\thetheo{\ref{sec:two}.\arabic{theo}}
\begin{theo}
test
\end{theo}
}

\section{Test Section One}
\label{sec:two}

\section{Test Section One}
\label{sec:three}

\end{document}