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CorollaryStatement: AI-adaptedProof: AI-adaptedprecheck passaudited 2026-09-01
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A finite flat module over a Noetherian ring is finite projective

Statement

Let R be a Noetherian commutative ring and let M be a finite flat R-module. Then M is finite projective.

Facts & Assumptions

Given: A Noetherian commutative ring R and a finite flat R-module M.

[L1]

A finite flat module over a Noetherian local ring is free (A finite flat module over a local ring is free).

[L3]

Flat modules satisfy the equational criterion for every finite family of relations (The equational criterion characterizes flat modules by lifting finite relations on generators).

[L4]

A module is projective exactly when every epimorphism onto it splits (Equivalent characterizations of projective modules).

Proof

technique · direct
1.1

For every maximal ideal mR, the localization Mm is finite and flat over the Noetherian local ring Rm. By [L1], it is free.

L1given
1.2

By [L2], choose a finite presentation RmARnπM0, writing xi=π(ei). Apply [L3] simultaneously to the finitely many relations given by the columns of A. It supplies elements yjM and coefficients bij such that xi=jbijyj and every relation among the xi is also a relation among the corresponding coefficient columns. Choose lifts zjRn of the yj and define s0(ei)=jbijzj. Then πs0=π, while the relation condition says that s0 kills im(A)=ker(π). Hence s0 descends to s:MRn with πs=1M. Thus the presentation epimorphism splits, so [L4] makes M projective. Since M is finite, it is finite projective.

L2L3L4construct
2.1

Thus every finite flat module over a Noetherian ring is finite projective.

algebra

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