# Restart theorem counter to 1 by book chapter

I am using \documentclass{book} with the amsmath and amsthm packages. Each chapter is a standalone paper, so I want the theorems in each chapter to begin with 1. How do I do this?

Here's a MWE.

\documentclass[printer]{book}
\usepackage{amsmath}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}

\begin{document}
\chapter{Blahblah}
\begin{theorem}{This is Theorem 1 as it should be}
\end{theorem}

\begin{theorem}{This is Theorem 2 as should be}
\end{theorem}

\chapter{Blibli}

\begin{theorem}{I need this to be Theorem 1!}
\end{theorem}

\end{document}

• Did you try \newtheorem{theorem}{Theorem}[chapter]
– Tom
Jun 6 at 17:53
• Take a look at the chngcntr package which deals in general with this sort of thing. Jun 6 at 18:38
• @Tom This adds the chapter number before the theorem number, to produce Theorem 1.1 and Theorem 1.2, and then Theorem 2.1. I want to suppress the chapter number. Jun 6 at 22:28
• @Peter Wilson I took a look but I'm too much of a newbie to know what to do with it! Jun 6 at 22:30
• OK, I think I found a simple solution. Writing \setcounter{theorem}{0} at the beginning of the new chapter seems to reset the theorem numbers! Jun 6 at 22:36

If don't want manually write \setcounter{theorem}{0} every time, you could do this:

\documentclass{book}
\usepackage{amsmath}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}[chapter]
\renewcommand{\thetheorem}{\arabic{theorem}}

\begin{document}
\chapter{Blahblah}
\begin{theorem}
This is Theorem 1 as it should be
\end{theorem}
\begin{theorem}
This is Theorem 2 as should be
\end{theorem}

\chapter{Blibli}
\begin{theorem}
I need this to be Theorem 1!
\end{theorem}
\end{document}


This does what you want using the chngcntr package. Please read the package manual again.

% thmcntrprob.tex  SE 646889

\documentclass[printer]{book}
\usepackage{amsmath}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}

\usepackage{chngcntr}
\counterwithin*{theorem}{chapter}

\begin{document}
\chapter{Blahblah}
\begin{theorem}{This is Theorem 1 as it should be} \end{theorem}

\begin{theorem}{This is Theorem 2 as should be} \end{theorem}

\chapter{Blibli}

\begin{theorem}{I need this to be Theorem 1!} \end{theorem}

\end{document}

• chngcntr is no longer needed. Jun 8 at 17:14
• @egreg Why is chngcntr no longer needed? Perhaps it is now part of basic LaTeX; if so how does anyone know about it? Jun 9 at 17:46
• It's described in one of the ltnews Jun 9 at 19:35