伯克利
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AcardinalκisaBerkeleycardinal,ifforanytransitivesetMwithκ∈Mandanyordinalα<κthereisanelementaryembeddingj:MMwithα<critj<κ.ThesecardinalsaredefinedinthecontextofZFsettheorywithouttheaxiomofchoice. TheBerkeleycardinalsweredefinedbyW.HughWoodininabout1992athisset-theoryseminarinBerkeley,withJ.D.Hamkins,A.Lewis,D.Seabold,G.HjorthandperhapsR.Solovayintheaudience,amongothers,issuedasachallengetorefuteaseeminglyover-stronglargecardinalaxiom.Nevertheless,theexistenceofthesecardinalsremainsunrefutedinZF. IfthereisaBerkeleycardinal,thenthereisaforcingextensionthatforcesthattheleastBerkeleycardinalhascofinalityω.ItseemsthatvariousstrengtheningsoftheBerkeleypropertycanbeobtainedbyimposingconditionsonthecofinalityofκ(Thelargercofinality,thestrongertheoryisbelievedtobe,uptoregularκ).IfκisBerkeleyanda,κ∈MforMtransitive,thenforanyα<κ,thereisaj:MMwithα<critj<κandj(a)=a. Acardinalκiscalledproto-BerkeleyifforanytransitiveMκ,thereissomej:MMwithcritj<κ.Moregenerally,acardinalisα-proto-BerkeleyifandonlyifforanytransitivesetMκ,thereissomej:MMwithα<critj<κ,sothatifδ≥κ,δisalsoα-proto-Berkeley.Theleastα-proto-Berkeleycardinaliscalledδα. WecallκaclubBerkeleycardinalifκisregularandforallclubsCκandalltransitivesetsMwithκ∈Mthereisj∈E(M)withcrit(j)∈C. WecallκalimitclubBerkeleycardinalifitisaclubBerkeleycardinalandalimitofBerkeleycardinals. Relations IfκistheleastBerkeleycardinal,thenthereisγ<κsuchthat(Vγ,Vγ 1)ZF2 “ThereisaReinhardtcardinalwitnessedbyjandanω-hugeaboveκω(j)”(Vγ,Vγ 1)ZF2 “ThereisaReinhardtcardinalwitnessedbyjandanω-hugeaboveκω(j)”. Foreveryα,δαisBerkeley.ThereforeδαistheleastBerkeleycardinalaboveα. Inparticular,theleastproto-Berkeleycardinalδ0isalsotheleastBerkeleycardinal.
IfκisalimitofBerkeleycardinals,thenκisnotamongtheδα. EachclubBerkeleycardinalistotallyReinhardt. TherelationbetweenBerkeleycardinalsandclubBerkeleycardinalsisunknown. IfκisalimitclubBerkeleycardinal,then(Vκ,Vκ 1)“ThereisaBerkeleycardinalthatissuperReinhardt”.Moreover,theclassofsuchcardinalsarestationary. ThestructureofL(Vδ 1) IfδisasingularBerkeleycardinal,DC(cf(δ) ),andδisalimitofcardinalsthemselveslimitsofextendiblecardinals,thenthestructureofL(Vδ 1)issimilartothestructureofL(Vλ 1)undertheassumptionλi.e.thereissomej:L(Vλ 1)L(Vλ 1).Forexample,Θ=ΘL(Vδ 1)Vδ 1,thenΘisastronglimitinL(Vδ 1),δ isregularandmeasurableinL(Vδ 1),andΘisalimitofmeasurablecardinals.