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TheoremStatement: AI-adaptedProof: AI-adaptedprecheck passaudited 2026-09-01
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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 (R,m) be a Noetherian local ring and let M be a finite flat R-module. Then M is free.

Facts & Assumptions

Given: A Noetherian local ring (R,m) and a finite flat R-module M.

[L1]

If lifts of residue classes generate M/mM, they generate M (Assuming the Axiom of Choice, generators modulo an ideal in the Jacobson radical lift to generators).

[L2]

The equational criterion characterizes flatness by lifting finite relations (The equational criterion characterizes flat modules by lifting finite relations on generators).

Proof

technique · direct
1.1

Choose elements x1,,xrM whose residue classes form a basis of the vector space M/mM. By [L1], they generate M. Thus there is a surjection φ:RrM sending the ith standard basis vector to xi.

L1givenchoose
1.2

Let K=kerφ. Because R is Noetherian and Rr is finite, [L3] makes K finitely generated. Tensoring 0KRrM0 with the residue field k=R/m remains exact because M is flat, so 0K/mKkrM/mM0. The last map is an isomorphism by the choice of the xi, hence K/mK=0.

L3algebra
1.3

Nakayama now gives K=0. Thus φ is an isomorphism and MRr is free. The equational criterion [L2] explains why no hidden relation survives once the residue-field relations vanish.

L2algebra
2.1

Therefore every finite flat module over a Noetherian local ring is free.

algebra

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Dependency tree · two levels

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