Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/iii/paper-101/4/e/solution
Past exam of the mathematics course of the University of Cambridge 2018 iii Paper 101 4 e Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-05
No. Take over a field , and inside takeThis is a subring containing : its elements have arbitrary constant coefficient in as polynomials in , and every positive-degree coefficient is divisible by ; that condition is preserved by sums and products. The ring is Noetherian. Indeed, a nonzero ideal of has an element of least degree, and division by shows that every element of the ideal is a multiple of . Thus is a principal ideal domain, and its ideals are finitely generated.
In , let . These form an ascending chain. We show that . If it belonged, we could writeCancel in the integral domain , then apply the evaluation homomorphism . Every becomes a scalar in , so this would givewhich is impossible by comparison of degrees. Hence for every , andThis is a constant-plus-ideal non-Noetherian subring construction: the restricted coefficients prevent a finite list of positive powers of from generating all the required powers.
New to topics? Read the docs here!