For a countable set S and a natural number k, the set of k-tuples in S, the maps from [k] to S, is countable.
In the setting of Routine Facts Used Without Citation: Arithmetic, Maps, Predicativity and Indices, let be a countable set and let , the set of natural numbers. Let be the set of -tuples in , whose elements are the maps by Tuples in a Set: the Set of n-Tuples and Their Components (ii).
is countable.
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