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PCF theory
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'''PCF theory''' is the name of a [[mathematics|mathematical]] theory, introduced by Saharon {{harvs|txt|last=Shelah|year=1978}}, that deals with the [[cofinality]] of the [[ultraproduct]]s of [[ordered set]]s. It gives strong upper bounds on the cardinalities of [[power set]]s of [[regular cardinal|singular]] [[cardinal number|cardinals]], and has many more applications as well. The abbreviation "PCF" stands for "possible [[cofinality|cofinalities]]".
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