Problem with qedhere

Question

In a paper I'm writing, I end a proof with a displayed equation that has some cases in it. The relevant code looks something like the following:

\[f(x) = \begin{cases}
2x & \text{if $x<0$}\\
7  & \text{if $x=0$}\\
1-x & \text{if $x>0$}
\end{cases} \]
\end{proof}

What I want to happen is for the amsthm "tombstone" to be placed on the right-hand side of the page aligned with the "1-x" line. I think this should be possible by placing a \qedhere in an appropriate place, but I can't seem to figure out where (I've tried all the obvious choices, and they all give me incorrect behavior).

Thanks for any help!

Answer 1

This can be achieved using the ntheorem package. This package handles endmarks properly. Note, you should specify the amsmath and amsthm options if you use amsmath and the proofenvironment, further thmmarks to get endmarks.

Here's a complete example, placing the qed symbol at the same height like 1-x, like desired by you:

\documentclass{article} 
\usepackage{amsmath}
\usepackage[amsmath,amsthm,thmmarks]{ntheorem}
\begin{document}
\begin{proof}
\[ f(x) = \begin{cases}
     2x & \text{if $x<0$}\\
     7  & \text{if $x=0$}\\
    1-x & \text{if $x>0$}
\end{cases} \]
\end{proof}
\end{document}

Output:

You could use further features of ntheorem to customize the proof layout and to choose a qed symbol. The ntheorem documentation may tell you more.