Automatically QED in LaTeX's newtheorem command

Question

As the title says I need to define a QED symbol automatically (preferable a colorbox) at the end of my custom newtheorem.

Any suggestions would be appreciated.

Answer 1

I'm not sure what you mean. If you use the package amsthm you have the command \qed to put a QED wherever you want.

You can customize the \qed by changing the command \qedsymbol. So you would do something like

\usepackage{amsthm}
\renewcommand{\qedsymbol}{$\heartsuit$}
\newtheorem{thm}{Theorem}
\newenvironment{\mythm}{\begin{\thm}}{\qed \end{thm}}
...
\begin{mythm}
blah
\end{mythm}

This if you want the QED symbol to be the same that appears in proofs. If not you can do simply

\newtheorem{thm}{Theorem}
\newenvironment{\mythm}{\begin{\thm}}{\hfill $\heartsuit$ \end{thm}}
...
\begin{mythm}
blah
\end{mythm}

Of course you can change \heartsuit with whatever you want.

Answer 2

An alternative is to use the ntheorem package with the options thmarks. that is \usepackage[thmarks]{ntheorem}.

You can then play with the settings to get what you want exactly...

Answer 3

Advice: don't do it. The placing of the QED symbol depends in a non-straightforward way on the contents of your text in your environment. For instance, Andrea's code will fail if your theorem ends with display math. You will probably need to place it by hand sometimes, and you are less likely to overlook things if you always do this.

Much better would be to have an environment that complains if the QED symbol has not been placed. You can do this by defining a qedused flag, say by \newififqedused, that is unset when you enter the environment (define a wrapper using \newenvironment as Andrea said), set in a wrapper around the QED symbol, and tested when you exit the environment. If the flag is still unset on exit, issue a \PackageWarning or \PackageError.

If you really want to try to get a qed environment that tries hard to do the right thing, regardless of what is in the environment, you'll need to look into the contents of \lastbox to try to determine where the QED symbol should go. If it is an hbox, you are fine, just place the QED as per Andreas' solution. If it is an mbox, then I think making an hbox containing this mbox followed by the QED symbol should work - I'm not sure, I don't usually mess about with the typesetting of maths. If it is a vbox, you need to look inside the structure of the vbox to find where it should go, which sounds hard - I can't think how to do that in pure Tex. In Luatex, I think that Hans Hagen's trick in The LuaTEX way: \framed might be adapted for this.