Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2019/iii/paper-121/2/iii/solution

The definable power set performs one definability step over the single structure , whereas the constructible power set contains subsets of created at arbitrarily late stages of the constructible hierarchy.
For the concrete case , there are only countably many first-order formulas and finite tuples of natural-number parameters, so is a countable set. In contrast, the constructible universe satisfies ZFC, and Cantor theorem makes its full power set uncountable inside . Consequently
so the two notions do not agree in general.

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