Issue on changing style in LaTeX

Question

Currently I have a TeX document, written using sig-alternate.cls class file. I need to change it and use llncs.cls. However, when I attempt to compile the document with the class changed, I get the following error:

! Undefined control sequence.
<recently read> \mathbb 

If I comment the corresponding line, it gives an error on the next one:

! Undefined control sequence.
<recently read> \rightarrowtail 

and so on, deprecating me of several mathematical symbols' usage, which seems strange to me - aren't \mathbb and \rightarrowtail standard keywords in TeX? And how can it be that some style file prevents me from using them? I have checked this on a simple example:

\documentclass{llncs}
\begin{document}
$\rightarrowtail$
\end{document}

This leads to an error, but when I change the documentclass to sig-alternate.cls, everything works well. Also, if I write a simple document with llncs.cls without usage of these symbols, everything is OK too. I will be very grateful for your help. Thanks in advance.

Answer 1

You have to use the amssymb package to use \mathbb and \rightarrowtail. It seems as if sig-alternate.cls loads this package, whereas llncs.cls does not.

From sig-alternate.cls:

[...]
\usepackage{epsfig}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsfonts}
[...]