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I am running into the error message !Undefined control sequence with the below explanation and can't figure out what I'm doing wrong. I am using Mactex and Texmaker. I am new to LaTeX and would really appreciate your help!

! Undefined control sequence.
\beamer@doifinframe ...\par \begin {equation*} \x
(k) = \left ( \begin {arra...
l.123 \end{frame}
The control sequence at the end of the top line
of your error message was never \def'ed. If you have
misspelled it (e.g., `\hobx'), type `I' and the correct
spelling (e.g., `I\hbox'). Otherwise just continue,
and I'll forget about whatever was undefined.
! Missing $ inserted.
<inserted text>
l.123 \end{frame}
I've inserted a begin-math/end-math symbol since I think
you left one out. Proceed, with fingers crossed.
! Missing } inserted


The code:




    \newenvironment{stepitemize}{\begin{itemize}[<+->]}{\end{itemize} }
    \newenvironment{stepenumeratewithalert}{\begin{enumerate}[<+-| alert@+>]}{\end{enumerate}}
    \newenvironment{stepitemizewithalert}{\begin{itemize}[<+-| alert@+>]}{\end{itemize} }
    \setbeamertemplate{navigation symbols}{}
    \setbeamertemplate{navigation symbols}{}
    \setfootline{ \hfill \insertframenumber}


    \x(k) = \left(
        \end{array} \right)

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beamer is a presentation-style document class that requires your content to be included in a frame environment. Structural elements (like \section, say) do not have to be included in a frame, but an equation* certainly would. So use \begin{frame}...\end{frame}. – Werner Jun 13 '13 at 20:52
The error message states that \x is undefined. It's most probably a typo for x, isn't it? – egreg Jun 13 '13 at 20:57
Welcome to TeX.SX! – mafp Jun 13 '13 at 21:01

\x is udefined. You probably wanted to have x(k) = \left( instead of \x(k) = \left( just before \begin{array}.

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