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[Merged by Bors] - feat: port RingTheory.Flat #3009

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1 change: 1 addition & 0 deletions Mathlib.lean
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Expand Up @@ -1279,6 +1279,7 @@ import Mathlib.RingTheory.Coprime.Ideal
import Mathlib.RingTheory.Coprime.Lemmas
import Mathlib.RingTheory.Finiteness
import Mathlib.RingTheory.Fintype
import Mathlib.RingTheory.Flat
import Mathlib.RingTheory.FreeRing
import Mathlib.RingTheory.Ideal.Basic
import Mathlib.RingTheory.Ideal.IdempotentFg
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84 changes: 84 additions & 0 deletions Mathlib/RingTheory/Flat.lean
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/-
Copyright (c) 2020 Johan Commelin. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Johan Commelin

! This file was ported from Lean 3 source module ring_theory.flat
! leanprover-community/mathlib commit 62c0a4ef1441edb463095ea02a06e87f3dfe135c
! Please do not edit these lines, except to modify the commit id
! if you have ported upstream changes.
-/
import Mathlib.RingTheory.Noetherian

/-!
# Flat modules

A module `M` over a commutative ring `R` is *flat*
if for all finitely generated ideals `I` of `R`,
the canonical map `I ⊗ M →ₗ M` is injective.

This is equivalent to the claim that for all injective `R`-linear maps `f : M₁ → M₂`
the induced map `M₁ ⊗ M → M₂ ⊗ M` is injective.
See <https://stacks.math.columbia.edu/tag/00HD>.
This result is not yet formalised.

## Main declaration

* `Module.Flat`: the predicate asserting that an `R`-module `M` is flat.

## TODO

* Show that tensoring with a flat module preserves injective morphisms.
Show that this is equivalent to be flat.
See <https://stacks.math.columbia.edu/tag/00HD>.
To do this, it is probably a good idea to think about a suitable
categorical induction principle that should be applied to the category of `R`-modules,
and that will take care of the administrative side of the proof.
* Define flat `R`-algebras
* Define flat ring homomorphisms
- Show that the identity is flat
- Show that composition of flat morphisms is flat
* Show that flatness is stable under base change (aka extension of scalars)
For base change, it will be very useful to have a "characteristic predicate"
instead of relying on the construction `A ⊗ B`.
Indeed, such a predicate should allow us to treat both
`A[X]` and `A ⊗ R[X]` as the base change of `R[X]` to `A`.
(Similar examples exist with `Fin n → R`, `R × R`, `ℤ[i] ⊗ ℝ`, etc...)
* Generalize flatness to noncommutative rings.

-/


universe u v

namespace Module

open Function (Injective)

open LinearMap (lsmul)

open TensorProduct

/-- An `R`-module `M` is flat if for all finitely generated ideals `I` of `R`,
the canonical map `I ⊗ M →ₗ M` is injective. -/
class Flat (R : Type u) (M : Type v) [CommRing R] [AddCommGroup M] [Module R M] : Prop where
out : ∀ ⦃I : Ideal R⦄ (_ : I.Fg), Injective (TensorProduct.lift ((lsmul R M).comp I.subtype))
#align module.flat Module.Flat

namespace Flat

open TensorProduct LinearMap Submodule

instance self (R : Type u) [CommRing R] : Flat R R :=
⟨by
intro I _
rw [← Equiv.injective_comp (TensorProduct.rid R I).symm.toEquiv]
convert Subtype.coe_injective using 1
ext x
simp only [Function.comp_apply, LinearEquiv.coe_toEquiv, rid_symm_apply, comp_apply, mul_one,
lift.tmul, Submodule.subtype_apply, Algebra.id.smul_eq_mul, lsmul_apply]⟩
#align module.flat.self Module.Flat.self

end Flat

end Module