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We formalise results from computability theory in the theorem prover Isabelle/HOL. 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.