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LemmaStatement: Literature-sourcedProof: AI-adaptedPipeline-generatedjudge pass (gpt-6.1-sol)
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Resolution of singularities is functorial under smooth morphisms

Statement

Assume the Axiom of Choice (The Axiom of Choice).

Let K be a field of characteristic zero, let Y be an integral finite-type K-scheme and let φ ⁣:Y′→Y be a smooth morphism of K-varieties, with both Y and Y′ connected of finite type over K (Smooth morphism of schemes, Integral schemes). Then the canonical resolutions of Resolution of singularities in characteristic zero are compatible with φ: the natural morphism φ~ ⁣:Y~′→Y~,Y~′=Y~×YY′, is smooth and makes the square with res⁡Y and res⁡Y′ commute, and the identification is canonical. In particular a smooth morphism is resolved by the base change of the resolution of its target, and, for y~∈Y~ with image y∈Y, the fibre of φ~ at y~ is Yy′×κ(y)κ(y~).

Facts & Assumptions

Given: The Axiom of Choice; a smooth morphism φ ⁣:Y′→Y of K-varieties of finite type over a field K of characteristic zero; and the canonical desingularizations res⁡Y′ ⁣:Y~′→Y′ and res⁡Y ⁣:Y~→Y of Resolution of singularities in characteristic zero.

[F1]

Resolution of singularities in characteristic zero, proof steps 2.2–3.1: the canonical resolutions are compatible with smooth base change; the proof uses the earlier ambient-extension and marked-ideal smooth-commutation results, the marked-ideal construction independently of this consequence.

[F2]

Smoothness survives base change and composition: smooth morphisms remain smooth under base change.

Proof

1.1F1given

Apply the already proved smooth comparison [F1] to φ. It gives the canonical isomorphism Y~′≅Y~×YY′ over Y′. Composing it with the first projection defines φ~ and makes the required square Cartesian, hence commutative. Naturality for composites and identities follows from the canonical comparisons in [F1].

2.1F1F2step 1.1∎

The projection Y~×YY′→Y~ is the base change of φ, so it is smooth by [F2]. For y~↦y, its fibre is Y′×YSpec⁡κ(y~)=Yy′×κ(y)κ(y~) by associativity of fibre products. Thus the base change resolves the source and has the stated fibres.

Depends on

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