Theorem numeration shift

I would like to numerate my theorems not by chapters but by shifted chapters in such a way that the theorems in chapter 2 would read Theorem 1.1, Theorem 1.2 and so on.

I need something along the lines of

\newteorem{thm}{Theorem}["chapter - 1"]


where I don't know the correct syntax for "chapter - 1".

• Welcome to TeX.SX! It's rather unclear why you want such a numbering that I find confusing. – egreg Aug 20 '13 at 13:02
• Maybe you need this weird theorem numbering because you haven't numbered the chapters the way you want to. Perhaps use \chapter* for an introduction or preface, so the real chapter numbers start at 1 and not 2. – Ethan Bolker Aug 20 '13 at 13:40
• Your readers will not thank you for this:-) – David Carlisle Aug 20 '13 at 15:19

I find this confusing for the reader; a simple way to get it is as follows:

\documentclass{report}
\newtheorem{thm}{Theorem}[chapter]
\renewcommand{\thethm}{%
\number\numexpr\value{chapter}-1\relax.\arabic{thm}%
}

\begin{document}
\chapter{No theorems}

\chapter{With theorems}

\begin{thm}
This is a theorem.
\end{thm}

\begin{thm}
This one too.
\end{thm}

\end{document}


While I cannot speak for the usefulness of such a numbering, a possible solution is using the aliascnt package:

\documentlcass{scrbook}

\usepackage{aliascnt}
\usepackage{ntheorem}

\newaliascnt{shiftchap}{chapter}
\newtheorem{thm}{Theorem}[shiftchap]

\begin{document}

\chapter{test}

\begin{thm}test\end{thm}

\begin{thm}test2\end{thm}

\chapter{test2}

\begin{thm}test 3\end{thm}

\end{document}


should provide the desired output.

In this case, an alias of the chapter counter is made and the display of that counter is one less than the value of the counter.

Even though I do not really understand the use you might have of such a functionality, here is how I would do:

\documentclass{book}

\usepackage{amsthm}

\newtheorem{thm}{Theorem}[chapter]

\newenvironment{mythm}{%
\begin{thm}}%
{%
\end{thm}

\begin{document}

\chapter{Introduction}

\chapter{First Part}

\begin{mythm}
Here is my theorem
\end{mythm}

\end{document}


The trick is to redefine the thm environment such as it decreases the chapter counter by 1 at its beginning and increases this counter by 1 at its end.