[Merged by Bors] - feat(CategoryTheory/Abelian/Preradical): introduce and characterize radicals

[farmanb]

Following Stenström, a preradical Φ is called radical if it coincides with its self colon. We encode this as the existence of an isomorphism Φ.colon Φ ≅ Φ. We then prove a basic characterization of radical preradicals in terms of the vanishing of Φ.r on Φ.quotient.

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• depends on: [Merged by Bors] - feat(CategoryTheory/Abelian/Preradical): add Stenström's colon construction for preradicals #35585
  

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PR summary 6e9770b320

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.CategoryTheory.Abelian.Preradical.Radical (new file)	836

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Declarations diff

+ Radical
+ instance (Φ : Radical C) : IsIso (Preradical.toColon Φ.obj Φ.obj) := Φ.property
+ isRadical
+ isRadical_iff_isIso
+ isRadical_iff_isZero
+ isZero

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---

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[mathlib-merge-conflicts]

This pull request has conflicts, please merge master and resolve them.

[farmanb]

-awaiting-author

[joelriou]

There is still one suggestion of modification.

[farmanb]

-awaiting-author

[robin-carlier]

Mostly style nitpicks on my side.

[farmanb]

-awaiting-author

[farmanb]

-awaiting-author

[joelriou]

Thanks!

bors merge

[mathlib-bors]

Pull request successfully merged into master.

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