Examples
- The set of all natural numbers which are indices of computable ordinals is a set which is not .
- The set of elements of Cantor space which are the characteristic functions of well orderings of is a set which is not . In fact, this set is not for any element of Baire space.
- If the axiom of constructibility holds then there is a subset of the product of the Baire space with itself which is and is the graph of a well ordering of Baire space. If the axiom holds then there is also a well ordering of Cantor space.
Read more about this topic: Analytical Hierarchy
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