My understanding is that the more general case of this (the countable union of countable sets) requires choice to prove. However in the case of ordinals, it should be provable without choice since there is an explicit well ordering on ordinals. Based on this, how does one go about constructing the explicit surjection needed to prove this?
2026-03-26 01:28:42.1774488522
The countable union of countable ordinals is countable, using ZF
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