\begin{frame}{Words} \begin{block}{} \emph{Word} = finite sequence of \emph{symbols} %($a,b,c,\ldots$) from an \emph{alphabet $\Sigma$}. \end{block} \begin{itemize} \item notation for symbols: $a,b,c,\ldots$ \item notation for words: $u,v,w,x,y,z$ \item $a\in\Sigma$ means $a$ is a symbol from the alphabet $\Sigma$ \end{itemize} \pause\bigskip \begin{block}{} We write $\lambda$ for the \emph{empty word}. \end{block} \pause \alert{\emph{Important: $\boldsymbol{\lambda}$ is not a letter of the alphabet\,!}} \pause\bigskip \begin{goal}{} In programming, words are called \emph{strings}. \medskip Then $\lambda$ is the empty string $"\,"$ (has length $0$). \end{goal} \end{frame}