Alphabeta Math
TheoremStatement: AI-adaptedProof: AI-adaptedverified 2026-09-24 (gpt-6-sol)
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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 (R,m) be a Noetherian local ring and let M be a finite flat R-module. Then M is free.

Facts & Assumptions

Given: The Axiom of Choice, a Noetherian local ring (R,m) and a finite flat R-module M.

[L1]

Under the assumed Axiom of Choice, if lifts of residue classes generate M/mM, they generate M; likewise a finite module K with K/mK=0 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).

[L2]

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).

[L3]

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

technique · direct
1.1L1givenchoose

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

1.2L2L3algebra

Let K=ker⁡φ. Because R is Noetherian and Rr is finite, [L2] makes K finitely generated. Tensoring 0→K→Rr→M→0 with the residue field k=R/m remains exact by [L3], so 0→K/mK→kr→M/mM→0. The last map is an isomorphism by the choice of the xi, hence K/mK=0.

2.1L1step 1.2algebra

Applying [L1] to the finite module K and the empty list of generators gives K=0. Thus φ is an isomorphism and M≅Rr is free.

3.1algebra∎

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

Depends on

Used by

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