Next | Topology of Data Types | 12 |

Let

*X*be the set of all strings over some alphabetCertainly

*X*is semidecidableCertainly the empty set is semidecidable

If

*A*and*B*are semidecidable, so is A ∩ BIf ℱ is an infinite family of semidecidable sets, ∪ℱ is a semidecidable set

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