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[Merged by Bors] - feat(category_theory/sites/{sheaf, sheafification}): monomorphisms of sheaves are precisely monomorphisms of presheaves#15932

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[Merged by Bors] - feat(category_theory/sites/{sheaf, sheafification}): monomorphisms of sheaves are precisely monomorphisms of presheaves#15932
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@jjaassoonn jjaassoonn commented Aug 8, 2022

  • monomorphisms of sheaves are precisely monomorphisms of underlying presheaves (when presheaves can be sheafified)
  • a morphism between sheaves is epic if the morphism between underlying presheaves is epic. (The converse is not always true)

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@jjaassoonn jjaassoonn added the awaiting-review The author would like community review of the PR label Aug 8, 2022
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I think these results are all done in one form or the other in mathlib.
Still, the lemmas in the morphism level should still be useful, but probably should go in
category_theory.sites.sheafification instead.

Comment thread src/topology/sheaves/epi_mono.lean Outdated
Comment thread src/topology/sheaves/epi_mono.lean Outdated
Comment thread src/topology/sheaves/epi_mono.lean Outdated
@jjaassoonn jjaassoonn requested a review from erdOne August 8, 2022 22:40
Comment thread src/category_theory/sites/sheaf.lean Outdated
Co-authored-by: Andrew Yang <36414270+erdOne@users.noreply.github.com>
@jjaassoonn jjaassoonn changed the title feat(topology/sheaves/epi_mono): monomorphisms of sheaves are precisely monomorphisms of presheaves feat(category_theory/sites/{sheaf, sheafification}): monomorphisms of sheaves are precisely monomorphisms of presheaves Aug 10, 2022
hom_equiv_naturality_right' := λ P Q R η γ, by { dsimp, rw category.assoc } }

instance Sheaf_to_presheaf_preserves_mono : (Sheaf_to_presheaf J D).preserves_monomorphisms :=
functor.preserves_monomorphisms_of_adjunction (sheafification_adjunction _ _)
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I think that providing the right adjoint instance as @erdOne suggested is the better approach.
The rest looks good to me.

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Thanks 🎉

bors merge

@leanprover-community-bot-assistant leanprover-community-bot-assistant added ready-to-merge All that is left is for bors to build and merge this PR. (Remember you need to say `bors r+`.) and removed awaiting-review The author would like community review of the PR labels Aug 10, 2022
bors Bot pushed a commit that referenced this pull request Aug 10, 2022
… sheaves are precisely monomorphisms of presheaves (#15932)

* monomorphisms of sheaves are precisely monomorphisms of underlying presheaves (when presheaves can be sheafified)
* a morphism between sheaves is epic if the morphism between underlying presheaves is epic. (The converse is not always true)
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bors Bot commented Aug 10, 2022

Pull request successfully merged into master.

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@bors bors Bot changed the title feat(category_theory/sites/{sheaf, sheafification}): monomorphisms of sheaves are precisely monomorphisms of presheaves [Merged by Bors] - feat(category_theory/sites/{sheaf, sheafification}): monomorphisms of sheaves are precisely monomorphisms of presheaves Aug 10, 2022
@bors bors Bot closed this Aug 10, 2022
@bors bors Bot deleted the jjaassoonn/epi_mono_sheaves branch August 10, 2022 14:59
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