##

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:

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

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

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

,

to set the body font to *<declarations>*, and

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:**

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`.

`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:**

`%` 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:

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