\begin{frame} \small \begin{definition} A \alert{Turing machine} is a tuple $\langle Q, q_0, F, \Sigma, \Box, \delta \rangle$ where: \begin{itemize} \item $Q$ is a finite set of states, \item $q_0 \in Q$ is the initial state, \item $F \subseteq Q$ is the set of final states, \item $\Sigma$ is a finite alphabet, \item $\Box \in \Sigma$ is the blank symbol, \item $\delta : (Q \setminus F) \times \Sigma \to Q \times \Sigma \times \{L,R\}$ is the \alert{transition function}. \end{itemize} \end{definition} \medskip\pause Turing machines work on a two-sided infinite tape with one read/write head: \turing{\Box,\Box,1,0,1,0,a,0,1,0,0,\Box,\Box}{q}{0} \end{frame}