# Indicator at end of theorem

I'm writing a text with some very long theorem-environments in it and I find the italicised theorems visually not pleasing when they are rather longish. So I'm using the definition theorem style most of the times. This however has disadvantage that it is pretty unclear when a theorem actually ends. I have tried the following:

\documentclass{book}
\usepackage{amsthm}
\parindent0pt

\theoremstyle{definition}
\newtheorem{theorem}{Theorem}
\newenvironment{thm}{\begin{theorem}}{\hfill$\diamond$\end{theorem}}

\begin{document}

\begin{thm} This is some long theorem. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.

\begin{enumerate}
\item Part A
\item Part B
\end{enumerate}

Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. \end{thm}

Here is some text not in the theorem anymore.

\begin{thm}
Here is some other theorem that ends with a list.
\begin{enumerate}
\item Part 1
\item Part 2
\end{enumerate}
\end{thm}

Some more text.

\end{document}


The result is:

I like the diamond indicator in theorem 1. It is not distracting and makes it very clear where the theorem actually ends. In theorem 2 however the position of the diamond looks a bit awkward.

What would you recommend in such situations?

• This looks pretty good.
– masu
Nov 21, 2013 at 8:45
• What about using the ntheorem package? Nov 21, 2013 at 8:50
• Instead of marking the end of theorem-like environment with a little (and, in my opinion, fairly easy to overlook) symbol, you may want to consider encasing the theorem environment inside a quotation environment. However, this might not be satisfactory if your theorems contain wide display-equation material that doesn't fit inside the narrower text block of a quotation environment.
– Mico
Nov 21, 2013 at 8:54
• I'd simply use italics. Not only it clearly delimits the statement, but it makes it easier to find them when browsing the document. Nov 21, 2013 at 10:01
• We sometimes use this in material written for non-mathematicians at universitylevel. Especially handy to mark the end of long running examples. For this we use ntheorem, quite easy. Another method is to color the theorems, here I'd recommend mdframed, it has a special \newtheorem-like macro to create these. Works quite well. Nov 21, 2013 at 14:28

You can use the QED mechanism that amsthm provides: define an appropriate symbol, push a QED at the beginning of the theorem, pop it at the end. You can then put \qedhere somewhere if you need the marker earlier, and the final pop will then do nothing.

\documentclass{book}
\usepackage{amsthm}
\parindent0pt

\newcommand{\thmsymbol}{$$\diamond$$}

\theoremstyle{definition}
\newtheorem{theorem}{Theorem}

\newenvironment{thm}{\begin{theorem}%
\renewcommand{\qedsymbol}{\thmsymbol}\pushQED{\qed}}%
{\popQED\end{theorem}}

\begin{document}

\begin{thm}
This is some long theorem. Lorem ipsum dolor sit amet, consetetur
sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore
et dolore magna aliquyam erat, sed diam voluptua. At vero eos et
accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren,
no sea takimata sanctus est Lorem ipsum dolor sit amet.
\begin{enumerate}
\item Part A
\item Part B
\end{enumerate}

Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam
nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam
erat, sed diam voluptua. At vero eos et accusam et justo duo dolores
et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est
Lorem ipsum dolor sit amet.
\end{thm}

Here is some text not in the theorem anymore.

\begin{thm}
Here is some other theorem that ends with a list.
\begin{enumerate}
\item Part 1
\item Part 2 \qedhere
\end{enumerate}
\end{thm}

Some more text.

\begin{proof}
Exactly.
\end{proof}

\end{document}