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A finite flat module over a local ring is free
Statement
Assume the Axiom of Choice (The Axiom of Choice). Let be a Noetherian local ring and let be a finite flat -module. Then is free.
Facts & Assumptions
Given: The Axiom of Choice, a Noetherian local ring and a finite flat -module .
Under the assumed Axiom of Choice, if lifts of residue classes generate , they generate ; likewise a finite module with vanishes by the same lifting corollary applied to the empty list (Assuming the Axiom of Choice, generators modulo an ideal in the Jacobson radical lift to generators).
Over a Noetherian ring, finite modules are finitely presented, so the kernel of a map from a finite free module to a finite module is finite (Over a Noetherian ring a module is Noetherian exactly when it is finitely generated, exactly when it is finitely presented).
A short exact sequence with flat quotient remains short exact after tensoring with any module (A short exact sequence with flat quotient remains short exact after tensoring).
Proof
Choose elements whose residue classes form a basis of the vector space . By [L1], they generate . Thus there is a surjection sending the th standard basis vector to .
Let . Because is Noetherian and is finite, [L2] makes finitely generated. Tensoring with the residue field remains exact by [L3], so The last map is an isomorphism by the choice of the , hence .
Applying [L1] to the finite module and the empty list of generators gives . Thus is an isomorphism and is free.
Therefore every finite flat module over a Noetherian local ring is free.
Depends on
- The Axiom of Choice
- Assuming the Axiom of Choice, generators modulo an ideal in the Jacobson radical lift to generators
- Over a Noetherian ring a module is Noetherian exactly when it is finitely generated, exactly when it is finitely presented
- A short exact sequence with flat quotient remains short exact after tensoring
Used by
- A residue-field basis lifts to a basis of a finite flat module over a local ring Example
- Finite projective complex for proper flat coherent cohomology Lemma
- Local support and index bound for the different of a curve map Lemma
- projective dimension from last nonzero betti number Lemma
- The square degree of a Mumford map Lemma
- Nonzero multiplication on an abelian variety is finite and faithfully flat Theorem
Dependency tree · two levels
19 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Stacks Project, Section 10.78: Finite projective modules (standard reference, not scraped)
- Mihnea Mustata, Graduate Commutative Algebra, §10 (standard reference, not scraped)