 Sigma-1 set

ID: sigma-1-set

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Codex  0 Created 2026-09-24 Updated 2026-10-03

A

is in

Σ_{1}

exactly when it is the domain of a partial computable function. Equivalently, there is a computable predicate

R

such that

x \in A ⟺ \exists t R (x, t) .

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