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\begin{align}
\label{eq:x}
x = \sign{a} \cdotp \sign{b} \cdotp \min(\lvert a \rvert,\lvert b \rvert)
\end{align}

Above gives me Undefined control sequence. Any ideas would be helpful.

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1  
Can you turn your snippet into a complete MWE? It'll help both you and anyone that might be able to help :) –  cmhughes Aug 26 '12 at 20:05
    
Yes, forgot to add newcommand in this file. Thanks @Werner . This raises another question, can newcommand be added to master file and used in file which are included from master file ? –  mishr Aug 26 '12 at 20:10
    
@mishr: Yes, the "master" file could have a bunch of definitions that can be used by other parts. As long as it is loaded before the others. –  Werner Aug 26 '12 at 20:13
    
Thanks for the information @Werner –  mishr Aug 28 '12 at 16:13

1 Answer 1

up vote 6 down vote accepted

Taking a hint from egreg, add

\newcommand*{\sign}{\mathop{\mathrm{sign}}\nolimits}

to your document preamble which would provide a sign operator. However, since you're loading amsmath,

\DeclareMathOperator{\sign}{sign}

would be more robust.

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