# Undefined control sequence for equation

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:

   \documentclass[notes=show,beamer,t,notheorems]{beamer}

\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathpazo}
\usepackage{hyperref}
\usepackage{subfigure}
\usepackage{tikz}

\setcounter{MaxMatrixCols}{10}

\newenvironment{stepenumerate}{\begin{enumerate}[<+->]}{\end{enumerate}}
\newenvironment{stepitemize}{\begin{itemize}[<+->]}{\end{itemize} }
\newtheorem{assumption}{Assumption}
\newtheorem{proposition}{Proposition}
\usetheme{CambridgeUS}
\numberwithin{figure}{section}
\useoutertheme{umbcfootline}
\numberwithin{figure}{section}
\input{tcilatex}
\renewcommand{\QTR}[2]{\frametitle{#2}}

\begin{document}

\begin{equation*}
\x(k) = \left(
\begin{array}{c}
x_1(k)\\
x_2(k)\\
\vdots\\
\end{array} \right)

\end{equation*}

-
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}`.