Conflict between amsmath and ntheorem, or, how I fell into an infinite troubleshooting loop

Question

There are similar questions on this site, but nothing quite fixed this problem for me.

When using amsmath with ntheorem I get either a missing end of theorem symbol (when my theorem ends with an equation) or my cross-referencing gets screwed up.

Here is a minimal example of the missing end of theorem mark:

\documentclass{article}

\usepackage{amsmath}
\usepackage[thmmarks,amsmath]{ntheorem}
\theoremsymbol{*}
\newtheorem{myDefinition}{Definition}

\begin{document}

\begin{myDefinition}
    I don't get my end of definition mark \ldots
    \begin{equation}
        x=y
        \label{test}
    \end{equation}
\end{myDefinition}

\ldots but the equation cross-reference works: \eqref{test}.

\end{document}

Here is a minimal example of the screwed-up cross-referencing (the argument amsmath has been omitted from the ntheorem package call):

\documentclass{article}

\usepackage{amsmath}
\usepackage[thmmarks]{ntheorem}
\theoremsymbol{*}
\newtheorem{myDefinition}{Definition}

\begin{document}

\begin{myDefinition}
    Now I get my end of definition mark \ldots
    \begin{equation}
        x=y
        \label{test}
    \end{equation}
\end{myDefinition}

\ldots but the equation cross-reference fails: \eqref{test}.

\end{document}

I'm probably missing something obvious.

Answer 1

I can verify your problem on an up-to-date TL 2012. I agree with @egreg that it doesn't look great, but if you'd like to stick to your guns, then you can use an \mbox{} to make things 'work'.

This trick also works when using the mdframed package with a frame that ends with an equation

\documentclass{article}

\usepackage{amsmath}
\usepackage[thmmarks,amsmath]{ntheorem}
\theoremsymbol{*}
\newtheorem{myDefinition}{Definition}

\begin{document}

\begin{myDefinition}
    I don't get my end of definition mark \ldots
    \begin{equation}
        x=y
        \label{test}
    \end{equation}
    \mbox{}
\end{myDefinition}
\ldots but the equation cross-reference works: \eqref{test}.

\end{document}

Answer 2

This works

\documentclass{article}

\usepackage{amsmath}
\usepackage[ntheorem]{empheq}
\usepackage[thmmarks,amsmath]{ntheorem}
\theoremsymbol{*}
\newtheorem{myDefinition}{Definition}

\begin{document}

\begin{myDefinition}
I don't get my end of definition mark \ldots
\begin{empheq}{equation}
x=y
\label{test}
\end{empheq}
\end{myDefinition}
\ldots but the equation cross-reference works: \eqref{test}.

\end{document}

Answer 3

Please try the examples with the "development" version 1.33 of ntheorem.

This version, although by now more than 1 year old is still a bit unstable an not sufficiently tested since I changed major things in the handling of skips - so please check the skips in your documents.