# How to get this theorem numbering to work?

I'm using the thmtools package to format my theorems. I want theorems to be labeled like: I.1.0, which refers to the first theorem of the first section of the first chapter. (Also, the section counter for chapter I should start at 5.)

(Sorry for the stupid question. I am sure that the answer to it is in the documentation, but I have been poring over it for the past hour, and honestly I can't find it.)

Thanks!

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According to the thmtools documentation, it seems like you're interested in \declaretheorem[numberwithin=section]{theorem}. However, this will start numbering the theorem as 1, not 0. Also, I don't know why "chapter I should start at 5"? Regardless, if you're after roman numerals for your chapters, \renewcommand{\thechapter}{\Roman{chapter}} does this. Otherwise, please provide more detail. –  Werner Sep 12 '11 at 5:37
Since you don't usually get to have anything starting from 0 anywhere in LaTeX, this is not trivially possible in thmtools. Maybe what you want re: starting from 5 is not to reset the section counter at all for each chapter, i.e. (chapter, section) should behave like (part, chapter)? –  Ulrich Schwarz Sep 12 '11 at 8:48

Including the explanations from the comment by Werner, I tried to figure out how a MWE would look like in your case. It is possible to set/modify the counters manually like this:

    \documentclass{book}

\usepackage{thmtools}

\declaretheorem[numberwithin=section]{theorem}

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

\begin{document}
\chapter{title}

I must confess, I also find it strange that you want to start numbering with 0, but why not! :) With the line \addtocounter{theorem}{-1} you can force that.