How can I enumerate theorems using letters for specific sections?

Question

For example, in the appendix, I want to enumerate theorems as Theorem A.1, Theorem A.2, etc... while in the text I enumerate theorems as Theorem 1, Theorem 2, etc...

How can I do this? I am using the amsthm package.

Edit: I am using the ieeeconf document class.

Answer 1

You just need to \renewcommand{\thetheorem} and, of course, reset the numbering.

\documentclass{IEEEconf}

\usepackage{amsthm}

\newtheorem{theorem}{Theorem}

\begin{document}

\section{First Section}
\begin{theorem}
    My theorem.
\end{theorem}

\section{Last Section}
\begin{theorem}
    My other theorem.
\end{theorem}

\section*{APPENDIX}
\renewcommand{\thetheorem}{A.\arabic{theorem}}
\setcounter{theorem}{0}

\begin{theorem}
    Theorem in appendix.
\end{theorem}

\begin{theorem}
    Another theorem in the appendix.
\end{theorem}

\end{document}

Note that the class is called IEEEconf. Using ieeeconf may work on case insensitive file systems such as Windows, but not generally.

Answer 2

I figured out a solution, but please, feel free to pose a more elegant solution.

I needed to 1) reset the section counter at the appendix, 2) change the numbering from \Roman to \Alph , and 3) add a counter to the appendix. Here is a minimal example:

\documentclass{ieeeconf}

\usepackage{amsthm}

\newtheorem{theorem}{Theorem}

\begin{document}

\section{First Section}
\begin{theorem}
    My theorem.
\end{theorem}

\section{Last Section}
\begin{theorem}
    My other theorem.
\end{theorem}

\setcounter{section}{1}
\renewcommand{\thesection}{\Alph{section}}
\counterwithin{theorem}{section}
\section*{APPENDIX}

\begin{theorem}
    Theorem in appendix.
\end{theorem}

\begin{theorem}
    Another theorem in the appendix.
\end{theorem}

\end{document}

Hopefully, this is helpful to someone.