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I think I may be having a specific issue due to changing the numbering of chapters with Roman (instead of Arabic) numerals, though I could be wrong on that. I have used

\newtheorem{definition}{Definition}[section]

to number definitions, theorems, etc. by section number. However, after compiling, I have e.g. "Theorem I.1.1" when I would like it to be "Theorem 1.1". It is my understanding that (with an 'ordinary' document, i.e. no Roman numerals) my code would do precisely this.

To effect the Roman numeral numbering of chapters, I have used

\renewcommand{\thechapter}{\Roman{chapter}}

Is this causing my problem? Thanks.

1 Answer 1

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It appears that you are using book or a similar document class, and you define the counter definition to be numbered within section.

However, as you can see from the following example, the counter "section" is by itself of the form "I.1", so your definition will begin with "I.1" instead just 1.

It is the expected behavior, since if you have two definitions in two different chapters, without the Roman numero, it is hard for readers to tell which is which when you are referring to them. You can, of course, modify the format of the counter definition via

\renewcommand{\thedefinition}{\arabic{section}.\arabic{theorem}}

with which you will get numbering of the form "1.1" instead of "I.1.1".


Below is a complete MWE demonstrate this.

\documentclass{book}

\usepackage{amsthm}

\newtheorem{theorem}{Theorem}[section]

\renewcommand{\thechapter}{\Roman{chapter}}


\begin{document}

\chapter{Some chapter}

\section{Some section}

\begin{theorem}
    Text.
\end{theorem}


\noindent\textbf{For demonstration:}

The counter \texttt{chapter}: \thechapter

The counter \texttt{section}: \thesection

The counter \texttt{theorem}: \thetheorem


% Modify the format of the counter "theorem"
\renewcommand{\thetheorem}{\arabic{section}.\arabic{theorem}}


\bigskip

\noindent\textbf{After modification:}


\begin{theorem}
    Text.
\end{theorem}


\noindent\textbf{For demonstration:}

The counter \texttt{chapter}: \thechapter

The counter \texttt{section}: \thesection

The counter \texttt{theorem}: \thetheorem

\end{document}

enter image description here

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  • Thanks, very clear and helpful explanation! Commented Feb 28, 2022 at 9:24

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