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[Merged by Bors] - feat(AlgebraicTopology): StrictSegal simplicial sets are quasicategories #19270

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@gio256 gio256 commented Nov 19, 2024

We prove that any simplicial set satisfying the StrictSegal condition introduced in #18499 is a quasicategory. This implies that the nerve of a category is a quasicategory.

We construct a solution to the lifting problem against hornInclusion n i as a spineToSimplex of the path in the image of the spine of the standard simplex.

To prove that this extension restricts to the appropriate map on the horn, we apply spineInjective, reducing the question of equality between two n-simplices in X to that of equality between the paths along their spines. The remainder of the proof is a straightforward application of basic properties relating spines of simplices to their faces.

Co-Authored-By: Johan Commelin and Emily Riehl


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@gio256 gio256 added the WIP Work in progress label Nov 19, 2024
@github-actions github-actions bot added the new-contributor This PR was made by a contributor with at most 5 merged PRs. Welcome to the community! label Nov 19, 2024
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github-actions bot commented Nov 19, 2024

PR summary 321de16000

Import changes for modified files

Dependency changes

File Base Count Head Count Change
Mathlib.AlgebraicTopology.SimplicialSet.StrictSegal 821 823 +2 (+0.24%)
Import changes for all files
Files Import difference
Mathlib.AlgebraicTopology.SimplicialSet.StrictSegal 2

Declarations diff

+ Path.map
+ horn.spineId
+ horn.spineId_map_hornInclusion
+ id
+ id_eq_objEquiv_symm
+ instance : Quasicategory (nerve C) := inferInstance
+ instance : Quasicategory X := by
+ map_interval
+ mkOfSucc_δ_eq
+ mkOfSucc_δ_gt
+ mkOfSucc_δ_lt
+ objEquiv_id
+ spineToSimplex_map
+ spine_δ_arrow_eq
+ spine_δ_arrow_gt
+ spine_δ_arrow_lt
+ spine_δ_vertex_ge
+ spine_δ_vertex_lt
+ standardSimplex.spineId

You can run this locally as follows
## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.


No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary
  • The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
  • The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

@github-actions github-actions bot added the t-topology Topological spaces, uniform spaces, metric spaces, filters label Nov 19, 2024
@gio256 gio256 changed the title [WIP] StrictSegal simplicial sets are quasicategories [WIP] feat(AlgebraicTopology): StrictSegal simplicial sets are quasicategories Nov 20, 2024
@gio256 gio256 added t-category-theory Category theory and removed t-topology Topological spaces, uniform spaces, metric spaces, filters labels Nov 20, 2024
@gio256 gio256 changed the title [WIP] feat(AlgebraicTopology): StrictSegal simplicial sets are quasicategories feat(AlgebraicTopology): StrictSegal simplicial sets are quasicategories Nov 22, 2024
@gio256 gio256 removed the WIP Work in progress label Nov 22, 2024
@joelriou joelriou added the awaiting-author A reviewer has asked the author a question or requested changes label Nov 23, 2024
@gio256 gio256 removed the awaiting-author A reviewer has asked the author a question or requested changes label Nov 24, 2024
@jcommelin jcommelin mentioned this pull request Nov 25, 2024
@gio256 gio256 added the awaiting-author A reviewer has asked the author a question or requested changes label Nov 25, 2024
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Thanks 🎉

If CI passes, please remove the label awaiting-CI and merge this yourself, by adding a comment bors r+.

bors d+

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mathlib-bors bot commented Nov 25, 2024

✌️ gio256 can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

@leanprover-community-mathlib4-bot leanprover-community-mathlib4-bot added delegated and removed awaiting-author A reviewer has asked the author a question or requested changes labels Nov 25, 2024
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gio256 commented Nov 25, 2024

Thank you for the careful review @joelriou @jcommelin.

bors merge

mathlib-bors bot pushed a commit that referenced this pull request Nov 25, 2024
…ies (#19270)

We prove that any simplicial set satisfying the `StrictSegal` condition introduced in #18499 is a quasicategory. This implies that the nerve of a category is a quasicategory.

We construct a solution to the lifting problem against `hornInclusion n i` as a `spineToSimplex` of the path in the image of the spine of the standard simplex. 

To prove that this extension restricts to the appropriate map on the horn, we apply `spineInjective`, reducing the question of equality between two `n`-simplices in `X` to that of equality between the paths along their spines. The remainder of the proof is a straightforward application of basic properties relating spines of simplices to their faces.

Co-Authored-By: [Johan Commelin](https://github.com/jcommelin) and [Emily Riehl](https://github.com/emilyriehl)
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mathlib-bors bot commented Nov 25, 2024

Pull request successfully merged into master.

Build succeeded:

@mathlib-bors mathlib-bors bot changed the title feat(AlgebraicTopology): StrictSegal simplicial sets are quasicategories [Merged by Bors] - feat(AlgebraicTopology): StrictSegal simplicial sets are quasicategories Nov 25, 2024
@mathlib-bors mathlib-bors bot closed this Nov 25, 2024
@mathlib-bors mathlib-bors bot deleted the nerve branch November 25, 2024 12:41
gio256 added a commit that referenced this pull request Nov 25, 2024
We add the file
`Mathlib/AlgebraicTopology/Quasicategory/StrictSegal.lean` and move the
proof (implemented in #19270) that any `StrictSegal` simplicial set is a
`Quasicategory`.
gio256 added a commit that referenced this pull request Nov 25, 2024
We add the file `Mathlib/AlgebraicTopology/Quasicategory/Nerve.lean` and
move the proof (implemented in #19270) that the nerve of a category is a
`Quasicategory`.
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