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Given a curve $\mathcal{L}$, define two functions of distance transform $\kappa_{+}$ and $\kappa_{-}$ on \mathbb{R}^{2} as
$\kappa_{+}(x) = d(x,\mathbb{R}^{2}\D), x \in \mathbb{R}^{2},                     (1)$
$\kappa_{-}(x) = d(x,\mathbb{R}^{2}\D), x \in \mathbb{R}^{2},                     (2)$

I get following error:

Undefined sequence
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2  
Hi Svaidya, [meta.tex.stackexchange.com/questions/1436/… to tex-sx!) It's usually best to provide your code in the form of a [meta.tex.stackexchange.com/questions/228/… as it really helps those who might be able to provide assistance. Welcome! –  cmhughes Oct 31 '12 at 16:24
    
Just to follow my previous comment- there's a danger that your question might get closed if it is not posed clearly; your question is very reasonable, but would benefit greatly from being turned into a Minimal Working Example –  cmhughes Oct 31 '12 at 16:49

2 Answers 2

You need to be careful to use mathmode correctly; in particular, \mathbb needs to be in mathmode.

You didn't specify what the macro \D does, but it looks as if would you like to use the align environment, as follows

\documentclass{article}

\usepackage{amssymb}
\usepackage{amsmath}
\newcommand{\D}{D}

\begin{document}
 Given a curve $\mathcal{L}$, define two functions of distance transform $\kappa_{+}$ 
 and $\kappa_{-}$ on $\mathbb{R}^{2}$ as 
 \begin{align}
 \kappa_{+}(x) & = d(x,\mathbb{R}^{2}\D), x \in \mathbb{R}^{2} \\
 \kappa_{-}(x) & = d(x,\mathbb{R}^{2}\D), x \in \mathbb{R}^{2}
 \end{align}
\end{document}
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\D might depend on a certain package, the \mathbb{R} without $ surrounded should not produce the mentioned error, but an missing $. Hence \D should be the cause for the error (though depending on the OPs packages in use). –  Ronny Oct 31 '12 at 16:24
    
@Ronny thanks for your input. Have you tried compiling \mathbb without $? :) –  cmhughes Oct 31 '12 at 16:26
    
well i was right stating, that the OPs error does not come from the math mode missing around \mathbb R, though the real error message then is of course Error: \mathbb allowed only in math mode. :) –  Ronny Oct 31 '12 at 16:45

Please always post complete documents so that your problem may be reproduced. However \mathcal is a standard LaTeX command but \mathbb requires a package (such as amssymb)

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