4.7.2 The ntheorem Package

4.7.2 The `ntheorem` Package

The ntheorem package provides nine predefined theorem styles, listed in Table 4.1. The default is plain. When you define a new theorem-like environment with \newtheorem, it is given the style currently in effect. You can change the current style with:

\theoremstyle{<style name>}

where <style name> is the name of the theorem style.

**Table 4.1:** Predefined Theorem Styles Provided by `ntheorem`
`plain`	Like the original LaTeX style
`break`	Header is followed by a line break
`change`	Like `plain` but header and number interchanged
`changebreak`	Combination of `change` and `break`
`margin`	Number is set in the margin
`marginbreak`	Like `margin` but header followed by a line break
`nonumberplain`	Like `plain` but without the number
`nonumberbreak`	Like `break` but without the number
`empty`	No number and no name. Only the optional argument is used in the header.

In addition to these styles, you can also use

\theoremheaderfont{<declarations>}

to set the header font to <declarations>, which should consist of font declaration commands such as \normalfont,

\theorembodyfont{<declarations>}

to set the body font to <declarations>, and

\theoremnumbering{<style>}

to set the appearance of the theorem number, where <style> may be one of: arabic, roman, Roman, alph, Alph, greek, Greek or fnsymbol. Remember that the above commands all need to be used before the new theorem-like environment is defined. For additional commands that affect the style of the theorems, see the ntheorem documentation [10].

Example:

% in the preamble: \theoremstyle{marginbreak} \theorembodyfont{\normalfont} \newtheorem{note}{Note}[chapter] % later in the document: \begin{note} This is a sample note. The number is in the margin. \end{note}

Result (the vertical line in the image below indicates the boundary of the text area and won't appear in the PDF):

If you use the standard package option to ntheorem, it will automatically define the following environments: Theorem, Lemma, Proposition, Corollary, Satz, Korollar, Definition, Example, Beispiel, Anmerkung, Bemerkung, Remark, Proof and Beweis.

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Unlike amsthm's proof environment, ntheorem's Proof environment appends its optional argument in parentheses, if present, to the proof title. (Recall from earlier that amsthm's proof environment uses its optional argument as a replacement for the default proof title.)

Example:

Listing 13:

% in the preamble:

\usepackage[standard]{ntheorem}

% later in the document:

\begin{Definition}[Tautology]\label{def:tautology}
A \emph{tautology} is a proposition that is always true for any value of its variables.
\end{Definition}

\begin{Definition}[Contradiction]\label{def:contradiction}
A \emph{contradiction} is a proposition that is always false for any value of its variables.
\end{Definition}

\begin{Theorem}
If proposition $P$ is a tautology then $\sim P$ is a contradiction, and conversely.
\begin{Proof}
If $P$ is a tautology, then all elements of its truth table are true (by Definition~\ref{def:tautology}), so all elements of the truth table for $\sim P$ are false, therefore $\sim P$ is a contradiction (by Definition~\ref{def:contradiction}).
\end{Proof}
\end{Theorem}

\begin{Example}\label{ex:rain}
``It is raining or it is not raining'' is a tautology, but ``it is not raining and it is raining'' is a contradiction.
\end{Example}

\begin{Remark}
Example~\ref{ex:rain} used De Morgan's Law $\sim (p \vee q) \equiv \sim p \wedge \sim q$.
\end{Remark}

Result:

Image showing typeset output click here for a more detailed description.

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