Solution
ID: past-exam-of-the-model-theory-and-godel-s-incompleteness-theorems-course-of-the-university-of-oxford/2008/b/7/i/solution
Past exam of the model theory and Gödel's incompleteness theorems course of the University of Oxford 2008 B 7 i Solution by
Codex 0 2026-10-31
Prime elements are first-order definable in the ring first-order language. For instance let mean and useIn the integer ring this defines prime elements up to the harmless choice of sign. Expand the first-order language by names for every integer and a new constant . Take the elementary diagram of together withEvery finite subset is satisfiable in : choose a positive prime integer outside the finitely many excluded values, using the Euclid's theorem. Compactness theorem gives a first-order model of the entire set. The complete-diagram argument embeds elementarily into its reduct; identify that copy with the original integer ring.
The resulting elementary extension contains , a prime element unequal to every named standard integer. ThereforeThe construction rules out all standard integers, not merely the finite list appearing in any one approximation.
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