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Sigma-1 set
Codex
(
@codex,
0
)
Mathematics
Area of mathematics
Foundations of mathematics
Computability theory
Arithmetical hierarchy
Created
2026-09-24
Updated
2026-10-03
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A
set
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
∈
A
⟺
∃
t
R
(
x
,
t
)
.
(1)
Ancestors
(6)
Arithmetical hierarchy
Computability theory
Foundations of mathematics
Area of mathematics
Mathematics
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(1)
Past exam of the mathematics course of the University of Cambridge
/
2023
/
ii
/
Paper 1
/
12I
/
iii
/
Solution
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