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\begin{document}

\author{Jian Xu\inst{1} \and Xingyuan Zhang\inst{1} \and Christian Urban\inst{2}}

\institute{PLA University of Science and Technology, China \and King's College London, UK}

\maketitle

\begin{abstract}

We formalise results from computability theory in the theorem prover

Isabelle/HOL. 

%This theorem prover is based on classical logic which

%precludes \emph{direct} reasoning about computability: every boolean

%predicate is either true or false because of the law of excluded

%middle. The only way to reason about computability in a classical

%theorem prover is to formalise a concrete model of computation.  

Following the textbook by Boolos et al, we formalise Turing machines

and relate them to abacus machines and recursive functions. We ``tie

the know'' between these three computational models by formalising a

universal function and obtaining from it a universal Turing machine by

our verified translation from recursive functions to abacus programs

and from abacus programs to Turing machine programs.  Hoare-style reasoning techniques allow us

to reason about concrete Turing machine and abacus programs. 

%Our theory can be

%used to formalise other computability results.

%We give one example about the computational equivalence of 

%single-sided Turing machines. 

%{\it we give one example about the undecidability of Wang's tiling problem, whose proof uses

%the notion of a universal Turing machine.}

\end{abstract}

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\end{document}

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