# Cross-referenced equation never becomes defined

I have a strange problem with cross-referencing my equations: no matter how often I run pdflatex they stay undefined. Other references do get resolved.

Is there something wrong with the way I label my equation? Pdflatex does not complain about it (other than reporting it is undefined when trying to reference it).

Edit: As requested below I made a minimal working example. From this I learned that the problem seems to be a conflict between the amsmath and ntheorem packages. Not including one of these solves the problem.

Consider for example the following example:

\documentclass{article}
\usepackage{amsmath}
\usepackage[thmmarks]{ntheorem}

\begin{document}

\section{My Section}
\label{sec:MySection}

$$\label{eq:MyEquation} a = b$$

Reference my section \ref{sec:MySection}.
Reference my equation \ref{eq:MyEquation}.

\end{document}


Which is rendered as:

• This is not a complete example. The easiest way to make it a complete document is to add \documentclass{article} and \begin{document} at the beginning, and \end{document} at the end. If you do that, do you still get a strange result? If not, what do you have to add from your original document to make a complete example to show the problem? – pst Mar 27 '14 at 12:29
• Welcome to TeX.SX! You can have a look at our starter guide to familiarize yourself further with our format. Please complete you code snippet to a minimal working example (MWE). As it stands when placed a simple document it does not produce the behaviour you describe. – Andrew Swann Mar 27 '14 at 12:55
• If I add an appropriate \documentclass declaration and the document environment, I get no problem. My advice is “look for a typo” in the label name. – egreg Mar 27 '14 at 12:57

You need the additional option amsmath for ntheorem, see "3.2.1 amsmath" of ntheorem's documentation.

\documentclass{article}
\usepackage{amsmath}
\usepackage[thmmarks,amsmath]{ntheorem}

\begin{document}
\setcounter{section}{4}
\section{My Section}
\label{sec:MySection}

$$\label{eq:MyEquation} a = b$$

Reference my section \ref{sec:MySection}.
Reference my equation \ref{eq:MyEquation}.

\end{document}


• And a useful further information is, that amsmath also provides \eqref to reference equations, i.e. it surrounds the number with brackets like (1). – Ronny Mar 27 '14 at 14:58