Stationarity of ordinals of prescribed cofinality
ID: stationarity-of-ordinals-of-prescribed-cofinality
For a regular infinite , with a regular uncountable cardinal number, is stationary. Build a strictly increasing continuous -sequence in any club set, and take its supremum. Closure places it in that club and the cofinality of an increasing ordinal supremum gives cofinality . In particular the two disjoint sets and show that the club filter on is not an ultrafilter.
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