Past exam of the commutative algebra course of the University of Oxford 2017 2 d Solution 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.
Past exam of the mathematics course of the University of Cambridge 2016 iii Paper 101 5 Solution Created 2026-10-03 Updated 2026-10-06
The Artin-Rees lemma states that, for a Noetherian ring , an ideal , a finitely generated module , and a submodule , there is such thatThus the filtration on induced from the I-adic filtration of is eventually determined by one of its terms.
For the proof, form the Rees ring and its Rees module:Since is a Noetherian ring, write . Then , so it is a Noetherian ring by the Hilbert basis theorem. Generators of in degree zero generate as a module over , so this is a Noetherian module.
The graded submoduleis therefore finitely generated. Choose homogeneous generators with for some . For , taking degree- components givesThe same generators show that the final sum is exactly . This proves the Artin-Rees lemma. If , one may simply take .
Now letIt is a submodule of the Noetherian module , hence a finitely generated module. Apply the Artin-Rees lemma with and . Since is contained in every (including ), its conclusion reduces toWrite generators as . There is a matrix with entries in such that . Multiplication by the adjugate of shows thatThe determinant is congruent to modulo , so it is for some . This determinant trick produces a single such element annihilating all of , and in particular annihilating each . If , use .
Conversely, if with , then . Iterating gives for every . Hence the exact description isIn a local ring with contained in the maximal ideal, every is a unit, so this also yields the usual vanishing form of the Krull intersection theorem.
For a failure without the Noetherian ring hypothesis, let and use the semigroup algebraIts elements are finite sums of formal monomials , with multiplication . Each ring in the union is a polynomial ring and the inclusions are injective, so is an integral domain. It is not a Noetherian ring, sinceIndeed the generator on the left is the square of the next generator, while division in the opposite direction would require a negative exponent, which is unavailable in .
Let be the ideal of elements with zero constant term, equivalentlyIt is proper because taking the constant term gives . Every generator is , with , so . Thus this is a nonzero idempotent ideal, with for every . TakeFor every , the constant term of is , so . Since is an integral domain and , we have . This gives all three requested objects and proves that no element of the required form annihilates .
Semigroup algebra 2026-10-06
For an additive semigroup with identity zero, the semigroup algebra has a -basis of formal symbols , , and multiplication . For it is the integral domain . Its positive-exponent ideal is a nonzero idempotent ideal, and the strictly ascending chain shows that the algebra is not a Noetherian ring.