Solution

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

No. Let
the successor cardinal of computed by . Then , and regards as a cardinal number. Because is transitive, , so externally
On the other hand , hence in the ambient universe. A corresponding bijection belongs to because its rank is below the inaccessible limit . Thus regards as equinumerous with and therefore not as a cardinal. This is an instance of cardinal nonabsoluteness in a small transitive model.

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