Recalling a theorem

Question

Suppose I have some theorem in the paper

Section 1

Theorem 1.1. Let ...

And then later in the paper I want to recall the theorem by reprinting it

Section 4

We recall Theorem 1.1:

Theorem 1.1. Let ...

What's the proper way to do this?

Answer 1

The thm-restate package which is part of thmtools offers a restatable environment. In the following example, I also use hyperref and cleveref (its \cref macro automatically adds the correct theorem type). See section 1.4 of the thmtools manual for details.

\documentclass{article}

\usepackage{thmtools}
\usepackage{thm-restate}

\usepackage{hyperref}

\usepackage{cleveref}

\declaretheorem[name=Theorem,numberwithin=section]{thm}

\begin{document}

\section{First}

\begin{restatable}[Goldbach's conjecture]{thm}{goldbach}
\label{thm:goldbach}
Every even integer greater than 2 can be expressed as the sum of two primes.
\end{restatable}

\section{Second}

We recall \cref{thm:goldbach}:

\goldbach*

\end{document}

Answer 2

If you need it only once or twice or so, you can locally in the repeating: 1) tweak theorem label printing, 2) add -1 to the theorem counter since it gets increased by the repeated theorem. The code:

\documentclass{article}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}[section]
\begin{document}

\section{First}

\begin{theorem}\label{mythm}
Let $x$ ...
\end{theorem}

\section{Second}

\begin{theorem}
This should be two.one
\end{theorem}

Let us recall the theorem from the first section.

\begingroup
\def\thetheorem{\ref{mythm}}
\begin{theorem}
Let $x$ ...
\end{theorem}
\addtocounter{theorem}{-1}
\endgroup

\begin{theorem}
This should be two.two
\end{theorem}

\end{document}