# Equation-reference with cleveref

I am using cleveref, which seems to be working fine, except for my equation references, which are referenced as ??.

My equation is:

$$\label{ex2eqn} Y_{t} = \epsilon_t + 1.8Y_{t-1} + 0.8Y_{t-2} + 0.2Y_{t-6} - 0.36Y_{t-7} + 0.16Y_{t-8}$$


and my reference is

This transformation is done by inserting $Y_{t-1}$ into \Cref{ex2eqn}, then $Y_{t-2}$ and so on.


A minimal working example is:

\documentclass[11pt]{article}
\usepackage{amsmath}
\usepackage[thmmarks]{ntheorem}
\usepackage{cleveref}
\begin{document}
$$\label{ex2eqn} e=mc^2$$
A reference: \Cref{ex2eqn}.
\end{document}

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If I supplement your code snippets to a full example, I can't comprehend the problem. Please provide a minimal example that clearly reproduces the described behaviour. – Thorsten Donig Oct 8 '11 at 12:55
You need to compile twice to have the references set. Did you do that? – Andro Oct 8 '11 at 13:19
I did. I am sorry I did not supply a minimal working example in the beginning. Now that I have, it seems the problem is rather odd, and has something todo with the combination of thmmarks and amsmath... – utdiscant Oct 8 '11 at 13:22

Load ntheorem with the amsmath (compatibility) package option.

\documentclass[11pt]{article}
\usepackage{amsmath}
\usepackage[amsmath,thmmarks]{ntheorem}
\usepackage{cleveref}
\begin{document}
$$\label{ex2eqn} e=mc^2$$
A reference: \Cref{ex2eqn}.
\end{document}

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I found that loading the ntheorem package (without the additional amsmath option) before the amsmath package also fixed it. – cmhughes Oct 8 '11 at 15:44
@cmhughes: I doubt that this won't break other things. See section 3.2.1 of the ntheorem manual. – lockstep Oct 8 '11 at 15:47
Thanks for the reference! – cmhughes Oct 8 '11 at 15:51