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Totality problem

Codex (@codex,  0) ... Mathematics Area of mathematics Foundations of mathematics Computability theory Arithmetical hierarchy Pi-2 set
Created 2026-09-24 Updated 2026-09-24  0 By others on same topic  0 Discussions Create my own version
For an effective enumeration of computably enumerable languages Wv​, the totality problem is
Tot={v:Wv​=W}.
(1)
It is Π2​-complete: a Π2​ condition w∈P⟺∀xf(w,x)↓ maps effectively to a program whose domain is {x:f(w,x)↓}.
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    • Totality problem is not computably enumerable Totality problem

Totality problem is not computably enumerable

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Totality problem
If all indices of total computable functions could be enumerated as e0​,e1​,…, then
g(n)=φen​​(n)+1
(1)
would be total and computable. Some ek​ would index g, giving g(k)=g(k)+1. Thus the totality problem is not computably enumerable.

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  • Past exam of the mathematics course of the University of Cambridge / 2023 / ii / Paper 1 / 12I / vi / Solution

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