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A finite flat module over a Noetherian ring is finite projective
Statement
Let be a Noetherian commutative ring and let be a finite flat -module. Then is finite projective.
Facts & Assumptions
Given: A Noetherian commutative ring and a finite flat -module .
A finite flat module over a Noetherian local ring is free (A finite flat module over a local ring is free).
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).
Flat modules satisfy the equational criterion for every finite family of relations (The equational criterion characterizes flat modules by lifting finite relations on generators).
A module is projective exactly when every epimorphism onto it splits (Equivalent characterizations of projective modules).
Proof
For every maximal ideal , the localization is finite and flat over the Noetherian local ring . By [L1], it is free.
By [L2], choose a finite presentation , writing . Apply [L3] simultaneously to the finitely many relations given by the columns of . It supplies elements and coefficients such that and every relation among the is also a relation among the corresponding coefficient columns. Choose lifts of the and define . Then , while the relation condition says that kills . Hence descends to with . Thus the presentation epimorphism splits, so [L4] makes projective. Since is finite, it is finite projective.
Thus every finite flat module over a Noetherian ring is finite projective.
Depends on
- A finite flat module over a local ring is free
- A module is flat if and only if all prime localizations are flat, equivalently all maximal localizations are flat
- Equivalent characterizations of projective modules
- Over a Noetherian ring a module is Noetherian exactly when it is finitely generated, exactly when it is finitely presented
- The equational criterion characterizes flat modules by lifting finite relations on generators
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
25 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, Lemma 10.78.2 (standard reference, not scraped)
- Mihnea Mustata, Graduate Commutative Algebra, §10 (standard reference, not scraped)