= Recursively inseparable-set proof of Tennenbaum theorem
Choose disjoint computably enumerable, recursively inseparable sets $A,B$. A nonstandard arithmetic model internally codes the elements enumerated into $A$ below a nonstandard stage by divisibility by standard primes. If the model's addition and multiplication were computable, division by each standard prime would decide the coded standard set. It would contain $A$ and avoid $B$, contradicting recursive inseparability.
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