How to create a figure with math content

Question

What's wrong with the following figure in the text? First, it says "amsmath: Unable to redefine math accent \vec." and then "Undefined control sequence" but neither is any helpful to me. It uses LLNCS (Lecture Notes for Computer Science), see http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0

\documentclass{llncs}
\usepackage{amsmath}
\title{Aussagenlogik}
\author{someone}
\institute{somewhere}

\begin{document}

\maketitle

\section{Einleitung}
\paragraph{...}

\section{Definition einer Aussage}
\paragraph{...
}

$\[X sei eine Aussage. Dann ist der Wahrheitswert W(X) wie folgt definiert:\\
W(A) = \begin{cases} wahr, \mbox{wenn X zutrifft}\\ falsch, \mbox{wenn X nicht zutrifft}
\end{cases}
\cite{Sie}\]$

\begin{thebibliography}{999}
  \bibitem {Sie} Bla Bla
\end{thebibliography}

\end{document}

Answer 1

You are using the math mode erronously. Inside $...$ the math contents are typeset as inline math and \[ .. \] is used for standalone display style math which gives a new line automatically without an eq. number. Also \text{...} is used to temporarily stop the math mode such that the contents are typeset in the regular text style. So

\documentclass{llncs}
\usepackage{amsmath}
\title{Aussagenlogik}
\author{someone}
\institute{somewhere}

\begin{document}

\maketitle

\section{Einleitung}
\paragraph{...}

\section{Definition einer Aussage}
\paragraph{...
}

X sei eine Aussage. Dann ist der Wahrheitswert W(X) wie folgt definiert:
\[W(A) = \begin{cases} wahr, \text{wenn X zutrifft}\\ falsch, \text{wenn X nicht zutrifft}
\end{cases}
\cite{Sie}\]

\begin{thebibliography}{999}
  \bibitem {Sie} Bla Bla
\end{thebibliography}

\end{document}