Past exam of the mathematics course of the University of Cambridge 2008 iii Paper 7 3 a Solution Created 2026-10-03 Updated 2026-10-14
Choose whose classes form a basis of the cotangent space of a local ring , and put . Spanning gives . Thus the quotient module satisfies . It is a finitely generated module, since is a Noetherian ring.
Here is the needed form of the Nakayama lemma, with proof. Choose generators of and write with . For the column vector , . Multiply by the adjugate to obtain . Since , this determinant is a unit in the local ring , so every . Therefore , proving . Conversely, any generators of must span its cotangent space, so at least are needed. HenceHere denotes the minimal number of generators of a module .
For the additional minimum, the maximal ideal is itself -primary, so it supplies an eligible ideal generated by elements. ConsequentlyThe minimum here is over primary ideals. A bound for every -primary ideal would be false. Indeed, for and , but is primary and needs three generators. To verify primaryness, if and , then cannot be a unit, so and . The three displayed monomials form a basis of , proving the lower bound of three, while they already generate . This illustrates that primary ideals can need more generators than the maximal ideal.