How can I get a line break in a theorem headline?

Question

I have a theorem title, which is longer then the page width. How do I get the title in two lines?

\begin{theorem}[this title is to long for one line]
the theorem
\end{theorem}

Answer 1

Can you provide more information on the document class and packages you are using? For example, the following code seems to cope with long titles and descriptions but may not be addressing your problem:

\documentclass{amsart}
\newtheorem{theorem}{Theorem}
\begin{document}
\begin{theorem}[this theorem has a long title that extends well over a line but is handled without any manual intervention on the author's partand even this description goes over a line]
body of the theorem
\end{theorem}
\end{document}

If you could provide a similar Minimal Working Example that shows your problem this would help others to comment on it more usefully than I have here.

Following on from your comment (and assuming you mean scrbook for the class then using the amsthm package may suffice, e.g.:

\documentclass[11pt,a4paper,twoside ,fleqn,headsepline]{scrbook}
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}
\begin{document}
\begin{theorem}[this theorem has a long title that extends well over a line but is handled without any manual intervention on the author's part and even this description goes over a line]
Body of the theorem
\end{theorem}
\end{document}

Answer 2

A solution for LaTeX default theorem environment:

\makeatletter
\renewcommand*{\@opargbegintheorem}[3]{\trivlist
  \item[\hskip \labelsep{\bfseries #1\ #2}] \textbf{(#3)}\ \itshape}
\makeatother

Here the macro \@opargbegintheorem is redefined, which originally contains

  \item[\hskip \labelsep{\bfseries #1\ #2\ (#3)}]\itshape}

so the optional theorem title has been moved to after the label instead if being within, so now line breaks are possible.

Test example:

\documentclass{book}
\newtheorem{theorem}{Theorem}
\makeatletter
\renewcommand*{\@opargbegintheorem}[3]{\trivlist
      \item[\hskip \labelsep{\bfseries #1\ #2}] \textbf{(#3)}\ \itshape}
\makeatother
\begin{document}
\begin{theorem}[This theorem has a very long title which requires a line break]
text
\end{theorem}
\end{document}

So much for information. I recommend using a capable theorem package such as amssthm or ntheorem which extends LaTeX's theorem capabilities.

Answer 3

As there have been some comments about providing a fix for ntheorem, I tried to provide one. It seems to work – at least in a quick test – but I am not sure whether it is 100% correct (for example it may break some internal conventions of some packages).

% https://tex.stackexchange.com/a/179917:
\newcommand*{\NLS}{%
    \par
    \nobreak
    \vspace{-\parskip}%
    \noindent
    \ignorespaces
}

\makeatletter
% From ‘ntheorem.sty’:
\newtheoremstyle{mybreak}%
{\item[\rlap{\vbox{\hbox{\hskip\labelsep \theorem@headerfont
                ##1\ ##2\theorem@separator}\hbox{\strut}}}]}%
{\item[\hskip\labelsep \theorem@headerfont%
    ##1\ ##2]\hspace*{-\labelsep}{\theorem@headerfont\ (##3)\theorem@separator}\NLS}
\makeatother

MWE:

\documentclass{book}
\usepackage[standard]{ntheorem}

% https://tex.stackexchange.com/a/179917:
\newcommand*{\NLS}{%
    \par
    \nobreak
    \vspace{-\parskip}%
    \noindent
    \ignorespaces
}

\makeatletter
% From ‘ntheorem.sty’:
\newtheoremstyle{mybreak}%\newtheoremstyle{break}%
{\item[\rlap{\vbox{\hbox{\hskip\labelsep \theorem@headerfont
                ##1\ ##2\theorem@separator}\hbox{\strut}}}]}%
{\item[\hskip\labelsep \theorem@headerfont%{\item[\rlap{\vbox{\hbox{\hskip\labelsep \theorem@headerfont
    ##1\ ##2]\hspace*{-\labelsep}{\theorem@headerfont\ (##3)\theorem@separator}\NLS}%##1\ ##2]\hspace*{-\labelsep}{\theorem@headerfont\ (##3)\theorem@separator}\hfill\penalty\@endparpenalty%##1\ ##2\ (##3)\theorem@separator}\hbox{\strut}}}]}
\makeatother

\theoremstyle{mybreak}
\renewtheorem{Theorem}{Theorem}

\usepackage[all]{nowidow}

% For testing:
\usepackage[english]{babel}
\usepackage{blindtext}
\blindmathtrue

\begin{document}
    \begin{Theorem}[This theorem has a very long title which requires a line break]
        \blindtext\par
        \blindtext
    \end{Theorem}
    \blindtext\par
    \blindtext
\end{document}