set an alphabetic counter for environment

Question

I have a \newtheorem environment of the form

\newtheorem{theorem}{Theorem}
\newtheorem{lemma}[theorem]{Lemma}

which follows the numbering of the section.

How can I change the numbering from arithmetic to alphabetic? In the middle of the document?

Answer 1

It would be better if you posted a full minimal working example as the answer might (but probably doesn't) depend on the package that you use for theorems etc. In any case, I think you need

\renewcommand\thetheorem{\alph{theorem}}

With this you obtain:

I am assuming that you only want to make the theorem counter alphabetic, and not the sections. Here is the full code (a MWE):

\documentclass{article}

\newtheorem{theorem}{Theorem}

\begin{document}

  \begin{theorem}first one \end{theorem}

  \renewcommand\thetheorem{\alph{theorem}}

  \begin{theorem}second one \end{theorem}

\end{document}