# [OS X TeX] Using \theoremstyle{}

Michael Sharpe msharpe at ucsd.edu
Sat Oct 23 16:33:25 EDT 2010

On Oct 23, 2010, at 1:19 PM, Don Green Dragon wrote:

> This is in response to messages from Michael Welsh about how one can avoid the use of italic font within the body of an environment introduced via the \newtheorem command. For example, Michael wrote:
>
>> If you replace this line with the two lines:
>> \theoremstyle{definition}
>> \newtheorem{defn}{Definition}[chapter]
>>
>> it will remove the annoying italic font, and allow you to use the defn environment correctly.
>
> I checked this out using the example source file "sampart.tex" that George Gratzer provides in a directory which here is in ~/Documents/Samples. By fiddling with the variations:
>
> \theoremstyle{plain}	\theoremstyle{definition}	\theoremstyle{remark}
>
> I was able to observe how things changed. Very good, but .... Gratzer uses the document class "amsart" whereas I am using the 'old' (I guess) LaTeX class "book"; i.e., I start with
>
> \documentclass [11pt,fleqn,leqno] {book}
>
> and the version \theoremstyle{definition} is rejected with the Console claiming:
> ================
> ./Mechanics.tex:38: Undefined control sequence.
> l.38 \theoremstyle
>                  {definition}	% suppresses the use of italic text within th...
>
> ? h
> The control sequence at the end of the top line
> of your error message was never \def'ed. If you have
> misspelled it (e.g., \hobx'), type I' and the correct
> spelling (e.g., I\hbox'). Otherwise just continue,
> and I'll forget about whatever was undefined.
> ================
>
> Is there some package that can be included so that \theoremstyle will be used? Since \theoremstyle must be an AMS gadget, I note that my prelude contains:
>
> \usepackage{amsmath}
> \usepackage{amssymb}
> \usepackage{latexsym}
> \usepackage{amsfonts}
> \usepackage[mathscr]{eucal}
> \usepackage{lmodern}
>
> In case  you are going to suggest that I use
>
> \documentclass [11pt,fleqn,leqno] {amsbook}
>
> well ..... I tried that, and it made one hell of a mess of my document --- mess from my point of view! I've spent a lot of time developing a 'book' template that I like and have no intention of switching to {amsbook}. It is not crucial that \theoremstyle be available but it worked so nicely that I hope someone will have a simple solution. :-)  People on this list are so incredibly helpful that I'm expecting a swift solution!  ;-)
>

It's in amsthm.sty, so write

\usepackage{amsthm}

Michael
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