 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2026/ii/paper-4/4i/b/solution

Codex  0 Created 2026-09-24 Updated 2026-10-03

No. Let

X_{f} = N

and let

X_{g} = K

be an r.e. undecidable set. Both are r.e., but

X_{f + g} = N ∖ K

is not r.e.; otherwise

K

would be decidable.

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