Appendix section numbering that mimicks theorem numbers?

Question

I have a document with several theorems ("\begin{restatable} \label{thmA}..."). After each theorem I have the statement "see proof in section \ref{sec:thmA:proof}".

So my appendix is now a long series of sections numbered in the usual way ("7.1, 7.2, ..."). However, I would like the section numbers to be the same as the theorem numbers, except for a prefixed "A".

For example, for theorems:

Theorem 1.1

Theorem 2.1

Theorem 2.2

I would like the appendix to look like:

A1.1. Proof of theorem 1.1

A2.1. Proof of theorem 2.1

A2.2. Proof of theorem 2.2

Here is a working example, which uses the thm-restate package to restate theorems in the appendix, although this is not crucial for the question:

\documentclass[]{article}

\usepackage{amsmath,amsthm}
\usepackage{thmtools,thm-restate}

\newtheorem{thm}{Theorem}[section]

\begin{document}

\section{A section in the body}

\begin{restatable}{thm}{thmA} \label{thmA}
Statement of the theorem.
\end{restatable}

The proof of theorem \ref{thmA} can be found in the appendix (see section \ref{sec:thmA:proof}).

\newpage
\appendix

The appendix starts here.

\section{Proof of theorem \ref{thmA}} \label{sec:thmA:proof}

\thmA*

\begin{proof}
Here goes the proof.
\end{proof}

\end{document}

In the previous example, the theorem is numbered Theorem 1.1., and the first section of the appendix is titled A. Proof of theorem 1.1.. I would like it to be A1.1., which includes the information of the theorem number.

Answer 1

This solution redefines the thm environment and adds the proof argument to it, which is later on written to the appendix section automatically.

It will also write a \section{Proof of Number...} to the appendix, which has to be included with \CollectProofs

It is not restatable at the moment!

\documentclass{article}%

\usepackage{blindtext}%
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amsthm}

\usepackage{letltxmacro}%
\usepackage{thmtools,thm-restate}

\theoremstyle{plain}% default
\newtheorem{thm}{Theorem}[section]




\newwrite\appendixcontent%

\AtBeginDocument{%
  \immediate\openout\appendixcontent=\jobname.proofs
}%

\AtEndDocument{%
  \immediate\closeout\appendixcontent%
}%


\newcommand{\CollectProofs}{%

\immediate\closeout\appendixcontent%

\renewcommand{\thesection}{A\arabic{thm}}

\IfFileExists{\jobname.proofs}{\input{\jobname.proofs}}{False}%
}%


\LetLtxMacro\StandardTheoremBegin\thm
\let\StandardTheoremEnd\endthm


\makeatletter
\renewenvironment{thm}[1]{%
\thmt@toks{}%
\refstepcounter{thm}%
\immediate\write\appendixcontent{%
  \string\renewcommand{\noexpand\thesection}{A\thethm}%
  \string\section{Proof of \thethm}%
    \string\begin{proof}%
    \string\unexpanded{#1}%
    \string\end{proof}%
}
\addtocounter{thm}{-1}%
\StandardTheoremBegin%
}{%
  \StandardTheoremEnd%
}%



\begin{document}



\section{A section in the body}


\begin{thm}{Nothing}
\blindtext
\end{thm}


\begin{thm}{Already proven}%
  On Brontosaurs -- Part 2\par
\blindtext[2]%
\end{thm}%

\section{Other Brontosaur theories -- by Misses Ann Elk}


\begin{thm}{See excavations}
By Misses Ann Elk\par
Brontosaurs are thin at one end, thick in the middle and thin again at the other end.
\end{thm}


\clearpage

\appendix

\CollectProofs


\end{document}

Answer 2

One possibility would be to define a new command \proofsection which temporarily redefines \thesection:

\newcommand{\proofsection}[1]{%
    \let\origsection\thesection
    \def\thesection{\Alph{section}\ref{#1}}
    \section{Proof of theorem \ref{#1}}
    \let\thesection\origsection
}

This may not be ideal and may need some tweaking, but I think it might be a start on how to tackle the problem.

Using that, your example would become:

\documentclass[]{article}

\usepackage{amsmath,amsthm}
\usepackage{thmtools,thm-restate}

\newtheorem{thm}{Theorem}[section]

\newcommand{\proofsection}[1]{%
    \let\origsection\thesection
    \def\thesection{\Alph{section}\ref{#1}}
    \section{Proof of theorem \ref{#1}}
    \let\thesection\origsection
}

\begin{document}

\section{A section in the body}

\begin{restatable}{thm}{thmA} \label{thmA}
Statement of the theorem.
\end{restatable}

The proof of theorem \ref{thmA} can be found in the appendix (see section \ref{sec:thmA:proof}).

\newpage
\appendix

The appendix starts here.

\proofsection{thmA}\label{sec:thmA:proof}

\thmA*

\begin{proof}
Here goes the proof.
\end{proof}

\section{A non-proof section in the appendix}

\proofsection{thmA}
this would be another proof section, referring to the proof section \ref{sec:thmA:proof}.

\end{document}