1
\newtheorem{lemma}{Lemma}[section]

\chapter{First chap}
\section{First sec}
\begin{lemma}
this one is correct.
\end{lemma}

\chapter{Second chap}
\begin{lemma}
This number is wrong.
\end{lemma}

\section{Second sec}
\begin{lemma}
this one is correct again.
\end{lemma}

Obviously I would like to have the lemma in the 2nd chapter before the Second sec to start at zero. But how?

  • The number is “2.0.1”; what do you expect it to be? – egreg Sep 5 '15 at 23:10
  • @egreg only in 2015 release – David Carlisle Sep 5 '15 at 23:23
  • @DavidCarlisle Hidden feature! – egreg Sep 5 '15 at 23:26
  • @egreg In my actual work there are some lemmas and theorems before the new chapter, so the first lemme in the new chapter has number i.e. 4.0.3 ; and this does not look fine – os23 Sep 5 '15 at 23:52
4

LaTeX < 2015: The number of the lemma is only reset, when section is increased, not chapter. \@addtoreset helps to reset lemma, if chapter is increased:

\newtheorem{lemma}{Lemma}[section]

\makeatletter
\@addtoreset{lemma}{chapter}
\makeatother

In the recent LaTeX release 2015/01/01, this is no longer necessary, because the counter reset is implemented in a transitive way.

Number format

If the section number 0 should be suppressed, then the following redefinition of \thelemma helps:

\renewcommand*{\thelemma}{%
  \ifnum\value{section}=0 %
    \thechapter
  \else
    \thesection
  \fi
  .\arabic{lemma}%
}
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