Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2023/iii/paper-116/3/c/solution

Yes. Start with the model constructed in part a. If has no internally strongly inaccessible cardinal above , put . Otherwise let be the least ordinal above that regards as strongly inaccessible, and put
In the second case because regards as inaccessible. The measure witnessing that is measurable has rank below , so it still belongs to . In both cases is a transitive set of cardinality , contains , and has no internally inaccessible ordinal strictly between and its height.
We verify absoluteness for every ordinal . If , then and both contain and therefore compute all subsets and functions relevant to strong inaccessibility in the same way. At , both models see a measurable cardinal and hence an inaccessible cardinal. Finally, if , then says that is not inaccessible by construction. The larger model cannot say that it is inaccessible, because strong inaccessibility is downward absolute to a transitive model of ZFC: any failure visible in the smaller model remains a failure in the larger one, while ambient inaccessibility would force internal inaccessibility. Hence “ is inaccessible” is absolute between and .

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