How to split an example into two parts?

Question

I am trying to have an example (using the theorem environent in amsmath) split in two parts:

Example 1 ...

Some more discussion

Example 1 (continued) ...

What is the best way to do this ?

The code I have so far is

\documentclass{article}

\usepackage{amsmath}
\newtheorem{example}{Example}

\begin{document}

Consider the following example:

\begin{example}
  $a = b + c$
\end{example}

Now, consider what happens if we add $d$ to $a$:

\begin{example}[continued] % I'd like to have this to output Example 1 (continued)
  $a + d$
\end{example}

\end{document}

Answer 1

I think you can keep most of your previous code. All you need to do is add a few instructions in the preamble -- specifically, load the ntheorem package and modify the default definition of theoremstyle (the latter is needed only if you want to see , continued rather than (continued)) -- and remember to issue the command \addtocounter{example}{-1} before typing \begin{example}[continued].

See the following MWE:

\documentclass{article}
\usepackage{amsmath}
\usepackage[amsmath]{ntheorem}
\makeatletter
\renewtheoremstyle{plain}%
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2\theorem@separator]}%
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2, ##3\theorem@separator]}
\makeatother
\newtheorem{example}{Example}

\begin{document}

\section{Some thoughts}
Consider the following example:
\begin{example}
$a = b + c$
\end{example}
Now consider what happens if we add $d$ to $a$:
\addtocounter{example}{-1}
\begin{example}[continued]
$a + d$
\end{example}
\end{document}

Edit: Following up on the suggestion of cmhughes, here's a version of the preceding MWE that's very similar but relieves the author of having to remember to type \addtocounter{example}{-1} before starting the example. This is achieved by creating a new environment I've named contexample; its style is identical to that of the example environment except that it appends ", continued" to the environment's header. Note that with this device, any example can be "continued" as often as one wishes.

\documentclass{article}
\usepackage{amsmath}
\usepackage[amsmath]{ntheorem}
\makeatletter
\renewtheoremstyle{plain}%
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2\theorem@separator]}%
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2, ##3\theorem@separator]}
\makeatother
\newtheorem{example}{Example}
\newenvironment{contexample}{
   \addtocounter{example}{-1} \begin{example}[continued]}{
   \end{example}}

\begin{document} 
\section{Some thoughts}
Consider the following example:
\begin{example}
$a = b + c$.
\end{example}
Now consider what happens if we add $d$ to $a$:
\begin{contexample}
$a + d$.
\end{contexample}
And if we wish to continue along these lines, we also find
\begin{contexample}
$x = y$.
\end{contexample}
\end{document}

Answer 2

Using the thmtools package as a front-end for amsmath (or ntheorem) you can easily achive what you need using the label=<key> and continues=<key> keys. A little example:

\documentclass{article}
\usepackage{amsthm}
\usepackage{thmtools}

\declaretheorem[style=definition]{example}

\begin{document}

\renewcommand\thmcontinues[1]{Continued}

\begin{example}[label=exa:cont]
This is an example.
\end{example}
\begin{example}[continues=exa:cont]
And here's the continuation of the example.
\end{example}

\end{document}

The text displayed is controlled by the \thmcontinues command which you can redefine according to your needs, as I did in my example code; by default it will display something like (continuing from p. <the_page>), where <the_page> is the page number of the first occurrence of the example.

Answer 3

Adapting cmhughes idea, but keeping the use of the \newtheorem command, you can simply make a new theorem type for continuations, and set the counter for that theorem to match the main example counter:

\documentclass{article}

\usepackage{amsmath}
\newtheorem{example}{Example}
\newtheorem{excont}{Example}
\renewcommand{\theexcont}{\theexample}

\begin{document}

Consider the following example:

\begin{example}
  $a = b + c$
\end{example}

Now, consider what happens if we add $d$ to $a$:

\begin{excont}[Continued]% I'd like to have this to output Example 1 (continued)
  $a + d$
\end{excont}

\end{document}

Answer 4

If the continuation always follows the main environment, this works

\documentclass{article}

\usepackage{amsthm}
\newtheorem{exampleinn}{Example}
\newtheorem*{example*}{Example \DO}
\newenvironment{example}
  {\xdef\DO{\number\numexpr\value{exampleinn}+1\relax\ (continued)}%
   \exampleinn}
  {\endexampleinn}

\begin{document}

Consider the following example:

\begin{example}
  $a = b + c$
\end{example}

Now, consider what happens if we add $d$ to $a$:

\begin{example*} % I'd like to have this to output Example 1 (continued)
  $a + d$
\end{example*}

\end{document}

The example* environment can be used any number of times and it will always refer to the main example. If the numbering of the example environment is more complex (say section.number), some more work is needed.

Answer 5

The following code defines two environments: example and examplecont. Note that only the example environment increments the counter.

\documentclass{article}

\usepackage{lipsum}% only needed for sample text

\newcounter{example}
\newenvironment{example}{%
\refstepcounter{example}%
{\bfseries Example \theexample}}{}

\newenvironment{examplecont}{%
{\bfseries Example \theexample} (cont\ldots)}{}

\begin{document}

\begin{example}
\lipsum[1]
\end{example}

\lipsum[2]

\begin{examplecont}
\lipsum[3]
\end{examplecont}
\end{document}

EDIT If you want a solution that uses an optional argument, here's one:

\documentclass{article}

\usepackage{lipsum}% only needed for sample text
\usepackage{ifthen}

\newcounter{example}
\newenvironment{example}[1][]{%
\ifthenelse{\equal{#1}{continued}}%
{%
{\bfseries Example \theexample (continued\ldots)\quad}%
}%
{%
\refstepcounter{example}%
{\bfseries Example \theexample \quad}%
}%
}{}

\begin{document}

\begin{example}
\lipsum[1]
\end{example}

\lipsum[2]

\begin{example}[continued]
\lipsum[3]
\end{example}
\end{document}

Answer 6

Starting from here, with just a few visual modifications:

\documentclass{article}
\usepackage{amsthm}
\begin{document}
\section{Original}

\newtheorem{exampleorig}{Example}

Consider the following example:

\begin{exampleorig}
  $a = b + c$
\end{exampleorig}

Now, consider what happens if we add $d$ to $a$:

\begin{exampleorig}[continued] % I'd like to have this to output Example 1 (continued)
  $a + d$
\end{exampleorig}

\section{Modified}

\newcounter{definition}
\newtheoremstyle{example}
{\topsep} {\topsep}%
{\upshape}% Body font
{}% Indent amount (empty = no indent, \parindent = para indent)
{\bfseries\scshape}% Thm head font
{.}% Punctuation after thm head
{1em}% Space after thm head (\newline = linebreak)
{}% Thm head spec

\theoremstyle{example}
\newtheorem{example}[definition]{Example}

\newtheoremstyle{example_contd}
{\topsep} {\topsep}%
{\upshape}% Body font
{}% Indent amount (empty = no indent, \parindent = para indent)
{\bfseries\scshape}% Thm head font
{.}% Punctuation after thm head
{1em}% Space after thm head (\newline = linebreak)
{\thmname{#1} \thmnumber{ #2}\thmnote{#3} (continued)}% Thm head spec

\theoremstyle{example_contd}
\newtheorem*{example_contd}{Example}

Consider the following example:

\begin{example} \label{ex:example1}
  $a = b + c$
\end{example}

Now, consider what happens if we add $d$ to $a$:

\begin{example_contd}[\ref{ex:example1}] % I'd like to have this to output Example 1 (continued)
  $a + d$
\end{example_contd}

\end{document}