Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/iii/paper-121/1/vi/b/solution

Let be the formula expressing cardinal number status:
Ordinalhood and the assertion that a given set is a bijection between fixed sets are each expressible by a bounded formula in set theory. If regarded as a non-cardinal ordinal, a witnessing smaller ordinal and bijection would remain present and valid in . Ordinalhood itself is absolute between the transitive models. Thus implies : this is downward absoluteness of cardinalhood.
For failure in the other direction, put . This ordinal belongs to , and with . Externally is countable, since and is countable. By the construction of , it contains a countability witness for , and hence a bijection . Therefore . This formula is downward absolute and fails to be upward absolute.

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