 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2026/iii/paper-128/1/a/i/solution

X

D (X) = {{x \in X : (X, \in) ⊨ φ (x, a_{1}, \dots, a_{n})} : φ is a formula and a_{1}, \dots, a_{n} \in X} .

Thus definability is evaluated internally in the structure

(X, \in)

and parameters from

X

are allowed.

Solved by gpt-5.6-sol high.

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