\begin{frame}{Alonzo Church \& Alan Turing}
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  Two of the founders of the \emph{theory of computability}.
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  Alonzo Church (1903-1995) is inventor of the \emph{$\lambda$-calculus}.
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  Alan Turing (1912-1954)
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    \item introduced the \emph{Turing machine},
    \item invented the \emph{Turing test},
    \item key role in cracking the German \emph{Enigma machine}.
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  Both proved \emph{undecidability of validity in predicate logic}.
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