Counters for claims

Question

I want to redefine the claim environment so that it will be numbered but I'm having a hard time with the counters.

I want the following:

\begin{theorem}
thm 1
\end{theorem}
\begin{proof}
  \begin{claim}
  blah 1
  \end{claim}

  \begin{claim}
  blah 2
  \end{claim}
\end{proof}

\begin{theorem}
thm 2
\end{theorem}
\begin{proof}
  \begin{claim}
  blah 3
  \end{claim}
\end{proof}

to produce:

Theorem 1: thm 1
Claim 1.1: blah 1
Claim 1.2: blah 2

Theorem 2: thm 2
Claim 2.1: blah 3

Answer 1

Here's a MWE that shows how to do this.

\documentclass{article}
\usepackage{amsmath,amssymb,amsthm}
\newtheorem{thm}{Theorem}
\newcounter{claimcounter}
\numberwithin{claimcounter}{thm}
\newenvironment{claim}{\stepcounter{claimcounter}{Claim \theclaimcounter:}}{}
\begin{document}

\begin{thm}
    Just testing.
\end{thm}
\begin{proof}
    \begin{claim}
        first
    \end{claim}

    \begin{claim}
        second
    \end{claim}

    \begin{claim}
        third
    \end{claim}
\end{proof}

\begin{thm}
    Just testing.
\end{thm}

\begin{thm}
    Just testing.
\end{thm}

    \begin{claim}
        first
    \end{claim}

    \begin{claim}
        second
    \end{claim}

    \begin{claim}
        third
    \end{claim}


\end{document}

The command \numberwithin allows the claims to be counted with respect to the theorem they are most closely associated with. \newenvironment allows you to create the environment for the claim---I haven't done much here other than issue out claim numbers.