[beamer] errore con usetheme

sto provando per la prima volta ad usare beamer compilando un .tex disponibile sul web ed ottengo questo errore:

! Undefined control sequence.
\usetheme

l.5 \usetheme
{default}
? x

Non capisco cosa possa essere, sembra un errore d’installazione ma ho installato e disinstallato il pacchetto almeno un paio di volte

il sogente è questo:
`% October 30, 2007
% Math 589

\documentclass{beamer}
\usetheme{default}
% \usetheme{Boadilla}
% \usetheme{Madrid}
% \usetheme{Montpellier}
% \usetheme{Warsaw}
% \usetheme{Copenhagen}
% \usetheme{Goettingen}
% \usetheme{Hannover}
% \usetheme{Berkeley}

% \usecolortheme{crane}
% \beamertemplatesolidbackgroundcolor{craneorange!25}

\title{Solving a quadratic equation}
\subtitle{a case study}

\author{Steven Hurder}

\institute[UIC] {University of Illinois at Chicago\\www.math.uic.edu/$\sim$hurder}

\date[May 10, 2007]
{Math 589 Presentation — October 30, 2007}

\begin{document}

\frame{\titlepage} % # 1
\section[Outline]{}
\frame{\tableofcontents}

\section{The Problem}

\frame % # 2
{
\frametitle{A pesky problem}

Your paycheck has been held up, and they keep asking,

\medskip

“Are you really a mathematician?''

\bigskip
\pause

How to convince them?

\medskip
\pause

What to do?

\medskip
\pause

And then the idea hits you – you'll show them you can solve a quadratic equation!

\medskip
\pause

If that doesn't convince the admin type, what will?

\vfill
}

\frame % # 2
{
\frametitle{Choosing a quadratic equation}

Now, it is only a matter to select a quadratic equation which will impress them.

\pause

\begin{enumerate}
\item $x^2 = 0$ \pause (nah, too obvious. it would be shameful if this worked)
\pause
\item $x^2 -2x +1 = 0$ \pause (more of the same)
\pause
\item $x^2 -3x -1 = 0$ \pause (sort of fancy… just right!)
\end{enumerate}

}

\section{Picturing the Solution}

\frame % 3
{
\frametitle{Grab your calculators:}

A picture may be worth a thousand words, but is it worth a thousand bucks?

\medskip
\pause

Let's try! If they buy this, we are done. So plot $y = x^2 -3x -1$

\pause

\begin{center}
\includegraphics[width=0.5\textwidth]{pix/quadratic.pdf}
\end{center}

\vfill

}

\frame % # 2
{
\frametitle{Not even close…}

“You want money for your one lousy graph?''

\pause

\bigskip

“Give the solution to 10 decimals, and we'll show you the money!''

\pause

\bigskip

“Oh, for ~ @\#\%\*\&\@ ~ sake!''
}

\section{Some Algebra}

\frame % 3
{
\frametitle{factor, factor, complete…}

\begin{eqnarray*}
0 & = & x^2 -3x -1 \\ \pause
0 & = & x^2 -3x + (-3/2)^2 – (3/2)^2 -1 \\ \pause
0 & = & (x – 3/2)^2 -9/4 – 4/4 \\ \pause
0 & = & (x – 3/2)^2 -13/4
\end{eqnarray*}

}

\frame % 3
{
\frametitle{Progress}

Now let's solve it:

\begin{eqnarray*}
0 = (x – 3/2)^2 -9/4 – 4/4 ~ & \Longrightarrow & ~ (x-3/2)^2 = 13/4 \\ \pause
& \Longrightarrow & ~ (x-3/2) = \pm \sqrt{13/4} \\ \pause
& \Longrightarrow & ~ x = 3/2\pm \sqrt{13/4}
\end{eqnarray*}

\pause

Think this is enough to get the money?

\medskip
\pause

Not likely…

}

\frame % 3
{
\frametitle{Pay Up!}

There are two solutions:

\bigskip
\pause

$ x = 3/2 + \sqrt{13/4}$, or
$$x = 3.302775637731994646559610633735247973125648286922623106355226528113583474146 505222602309541009245359$$

\bigskip
\pause

and $ x = 3/2 – \sqrt{13/4}$, or
$$x = -0.302775637731994646559610633735247973125648286922623106355226528113583474146505222602309541009245359$$

}

\section{The Formula}

\frame % 3
{
\frametitle{Mathematical Proof}

The final proof that we are Mathematicians?

\bigskip
\pause

Give them the Magic Formula,
$$ ax^2 + bx + c = 0 \Longrightarrow x = \frac{-b \pm \sqrt{b^2 – 4ac}}{2a}$$

\bigskip
\pause

and tell them to try this first next time…

}

\end{document}
`

Ho provato il tuo codice e ottengo l’output in .pdf, senza errori.
Mi vengono segnalati solo quattro warning.

`Package pgf Warning: This package is obsolete and no longer needed on input lin
e 13.

Package hyperref Warning: Option `pdfpagelabels' is turned off
(hyperref) because \thepage is undefined.

LaTeX Font Warning: Font shape `OT1/cmss/m/n' in size <4> not available
(Font) size <5> substituted on input line 31.

LaTeX Font Warning: Size substitutions with differences
(Font) up to 1.0pt have occurred.`

Che l’installazione sia “bacata”?

Ciao,
Antonio