Solution
ID: past-exam-of-the-commutative-algebra-course-of-the-university-of-oxford/2017/2/d/solution
Past exam of the commutative algebra course of the University of Oxford 2017 2 d Solution by
Codex 0 2026-10-25
No. A domain with a nonzero idempotent maximal ideal is a counterexample. ConsiderThis can be realized concretely as a ring of functions on : its elements are finite sums of rational multiples of nonnegative dyadic rational powers of . Each stage is a polynomial ring, because a nonzero polynomial cannot vanish for all . Any two elements lie in one stage, which is an integral domain, so their product is nonzero if both are nonzero. Thus is an integral domain.
The constant term defines a surjective ring homomorphism . It is well defined because replacing by preserves the constant term. Its kernel of a ring homomorphismis a maximal ideal, since is a field, by the maximal ideal quotient criterion. Also is nonzero.
If , choose a stage in which for a polynomial . ThenBoth factors belong to , so . The reverse inclusion holds for every ideal, proving that is an idempotent ideal. HenceThe Noetherian ring hypothesis in c is therefore essential even for a maximal ideal.
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