placement of QED symbol on numbered equation line

Question

This question is both on LaTeX and on mathematics writing style; I hope that's still apropriate here.

I have a proof that ends with a (right) numbered equation, and I'm pondering about where to put the QED symbol. The standard right margin position conflicts with the equation number, so what would be considered best style for the QED placement?

Secondly, I use the ntheorem package, but cannot place the QED symbol using \qedhere on this equation line. Is there a (simple) way to solve this?

Answer 1

I have the following two suggestions:

1. Reword your proof. Even if you merely state the obvious:

   ...
           x^n + y^n = z^n
   which completes the proof.
   

   or

   Proof. It follows trivially that for
           x^n + y^n = z^n
   there exist no solutions.
   

2. Place the QED symbol manually next to the equation and correct for horizontal alignment with \phantom or use \rlap. Hopefully this is not something that has to be done frequently.

   

   \documentclass[a4paper,11pt]{article}
   
   \usepackage{amsmath,amssymb}
   \usepackage[thmmarks,amsmath,amsthm,hyperref]{ntheorem}
   
   \newtheorem{theorem}{Theorem}[section]
   \begin{document}
   
   \begin{proof}
     foo.
   \end{proof}
   
   \begin{proof}
     It follows trivially that no solutions exist for
     \begin{equation*}\label{eq:fermat1}
       x^n + y^n = z^n.
       \refstepcounter{equation}
       \tag*{(\theequation)}
     \end{equation*}
   \end{proof}
   
   \begin{proof}
     It follows trivially that no solutions exist for
     \begin{equation}\label{eq:fermat2}
       x^n + y^n = z^n. \rlap{$\qquad \Box$}
     \end{equation}
   \end{proof}
   
   \end{document}
   

Answer 2

The following (somewhat dirty) solution seems to work:

\documentclass[a4paper,11pt]{article}

\usepackage{amsmath}
\usepackage[thmmarks,amsmath,amsthm,hyperref]{ntheorem}

\newtheorem{theorem}{Theorem}[section]

\begin{document}

\begin{proof}
  foo.
\end{proof}

\begin{proof}
  It follows trivially that no solutions exist for
  \begin{equation*}\label{eq:fermat}
    x^n + y^n = z^n.
    \refstepcounter{equation}
    \tag*{(\theequation)}
  \end{equation*}
\end{proof}

\end{document}

It places the QED symbol right below the equation number, which is not as ugly as I had thought it would be.

I must acknowledge @egreg's comment, though, that it is not perfect from a stylistic point of view.

Edit: Now a full MWE, and I noticed that I had defined

\newcommand{\qedhere}{}

for backward compatibility with amsmath because ntheorem automatically places the QED symbol.