Numbering claims

Question

I would like to have the claims in the proof of a given theorem or proposition numbered independently from those in a different theorem or proposition. This question has already been asked here: ntheorem: numbering of claim and its proof. However, I must be doing something wrong as it does not work for me. Typing

\documentclass[11pt]{amsart}

\theoremstyle{plain} \newtheorem{thm}{Theorem}[section] \newtheorem{prop}[thm]{Proposition} \newtheorem{claim}[thm]{Claim}

\begin{document}

\section{blah}

\begin{thm} 
\begin{claim} 
\end{claim} 
\end{thm}

\begin{prop} 
\begin{claim} 
\end{claim} 
\end{prop}

\end{document}

does not number claims within the theorem or proposition they refer to but sequentially to them. I tried also with \usepackage[thmmarks]{ntheorem}, but it gives me an error:

(Package ntheorem Error: Theorem style plain already defined.)

What am I doing wrong?

Answer 1

You are getting what you have asked for: claims numbered in the same sequence as theorems and propositions. If you want claims to be numbered within them, instead, you must replace

\newtheorem{claim}[thm]{Claim}

with

\newtheorem{claim}{Claim}[thm]

Here’s a MWE:

\documentclass[a4paper]{amsart}
\usepackage[T1]{fontenc} % unrelated to your question, but I'd recommend it

\theoremstyle{plain}
\newtheorem{thm}{Theorem}[section]
\newtheorem{prop}[thm]{Proposition}
\newtheorem{claim}{Claim}[thm]

\begin{document}

\section{Blah}

\begin{thm}
    Statement of theorem.
    \begin{claim}
        Statement of claim.
    \end{claim}
    More text.
    \begin{claim}
        Statement of another claim.
    \end{claim}
    End of theorem.
\end{thm}

\begin{prop}
    Statement of proposition.
    \begin{claim} 
        Statement of claim.
    \end{claim} 
    More text.
    \begin{claim}
        Statement of another claim.
    \end{claim}
    End of proposition.
\end{prop}

\end{document}

And here’s the output: