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Consider the following code:

\documentclass{amsart}
\begin{document}

\makeatletter
  \def\th@plain{
  \thm@notefont{}
  \itshape}
\makeatother

\theoremstyle{definition}
\newtheorem*{prop}{Proposition}

\begin{prop}[Optional Title]
  Content.
\end{prop}

\end{document}

I want to italicize the (bold) heading and (normal) optional title of the sample theorem (proposition).

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Would you be willing to use a package such as ntheorem or amsthm? –  Mico Mar 28 at 4:59
    
Sure, I just don't know what to do. –  fctaylor25 Mar 28 at 5:00
add comment

2 Answers 2

up vote 2 down vote accepted

The actual style that you want to update is the definition style (since you specify it), like so:

\documentclass{amsart}
\begin{document}

\makeatletter
  \def\th@definition{
  \thm@headfont{\itshape} % Heading font is italic
  \thm@notefont{} % Note is same as heading
  \itshape% Regular text is also italic
}
\makeatother

\theoremstyle{definition}
\newtheorem*{prop}{Proposition}

\begin{prop}[Optional Title]
  Content.
\end{prop}

\end{document}

result

Edit:

Note that the same result can be made with the article class by loading amsthm ... nearly all of the same code is used in both cases, so the same trick will apply in either setting. From the documentation:

Three document class files and one package file (amsthm.sty) are produced from this source. Most of the code of the amsthm package is used in all four derived files.

For example:

\documentclass{article}
\usepackage{amsthm}

\begin{document}

\makeatletter
  \def\th@definition{
  \thm@headfont{\itshape} % Heading font is italic
  \thm@notefont{} % Note is same as heading
  \itshape% Regular text is also italic
}
\makeatother

\theoremstyle{definition}
\newtheorem*{prop}{Proposition}

\begin{prop}[Optional Title]
  Content.
\end{prop}

\end{document}
share|improve this answer
    
Awesome, thank you. –  fctaylor25 Mar 28 at 5:05
    
See edit as well ; same effect is attainable with article (or other) class and \usepackage{amsthm} if that is what you prefer to use. –  cslstr Mar 28 at 5:08
    
That should come in handy. I appreciate the detailed answers. –  fctaylor25 Mar 28 at 5:10
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Here's a solution that employs the capabilities of the ntheorem package.

enter image description here

If you want the theorem's header separated from the body with, say, a "dot" (.), just add the instruction \theoremseparator{.} between the \theoremheaderfont and \newtheorem statements.

\documentclass{article}
\usepackage{ntheorem}
\theoremheaderfont{\textit}
\newtheorem{theorem}{Theorem}
\begin{document}
\begin{theorem}[Pythagoras]
Consider a right triangle with sides labelled $a$, $b$, and~$c$, and wlog assume that 
the longest side is labelled $c$. Then \[ a^2+b^2=c^2. \]
\end{theorem}
\end{document} 
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