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A finite flat module over a local ring is free
Statement
The standard theorem holds over arbitrary local rings; the proof written here is the Noetherian local case.
Let be a Noetherian local ring and let be a finite flat -module. Then is free.
Facts & Assumptions
Given: A Noetherian local ring and a finite flat -module .
If lifts of residue classes generate , they generate (Assuming the Axiom of Choice, generators modulo an ideal in the Jacobson radical lift to generators).
The equational criterion characterizes flatness by lifting finite relations (The equational criterion characterizes flat modules by lifting finite relations on generators).
Over a Noetherian ring, finite modules are finitely presented (Over a Noetherian ring a module is Noetherian exactly when it is finitely generated, exactly when it is finitely presented).
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, [L3] makes finitely generated. Tensoring with the residue field remains exact because is flat, so The last map is an isomorphism by the choice of the , hence .
Nakayama now gives . Thus is an isomorphism and is free. The equational criterion [L2] explains why no hidden relation survives once the residue-field relations vanish.
Therefore every finite flat module over a Noetherian local ring is free.
Depends on
- The equational criterion characterizes flat modules by lifting finite relations on generators
- 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
Used by
Dependency tree · two levels
17 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)