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[Merged by Bors] - feat(Topology): An open cover of a preconnected set has no isolated components#39691

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[Merged by Bors] - feat(Topology): An open cover of a preconnected set has no isolated components#39691
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@Raph-DG Raph-DG commented May 22, 2026

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In this PR we show that a family of open sets whose union is preconnected has the property that for any two (nonempty) sets U and V in the cover there is a sequence of sets in the cover U = U_0, U_1,..., U_n = V such that U_i \cap U_{i+1} is nonempty for all i.


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github-actions Bot commented May 22, 2026

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PR summary 0ec643349a

Import changes for modified files

No significant changes to the import graph

Import changes for all files
Files Import difference

Declarations diff

+ IsPreconnected.transGen_of_iUnion

You can run this locally as follows
## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.


No changes to strong technical debt.
No changes to weak technical debt.

@github-actions github-actions Bot added the t-topology Topological spaces, uniform spaces, metric spaces, filters label May 22, 2026
Comment thread Mathlib/Topology/Connected/Basic.lean

@b-mehta b-mehta left a comment

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Nice proof!

bors merge

@mathlib-triage mathlib-triage Bot added the ready-to-merge This PR has been sent to bors. label May 31, 2026
mathlib-bors Bot pushed a commit that referenced this pull request May 31, 2026
…omponents (#39691)

In this PR we show that a family of open sets whose union is preconnected has the property that for any two (nonempty) sets U and V in the cover there is a sequence of sets in the cover U = U_0, U_1,..., U_n = V such that U_i \cap U_{i+1} is nonempty for all i.

Co-authored-by: Raph-DG <raphaeldouglasgiles@gmail.com>
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mathlib-bors Bot commented May 31, 2026

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Pull request successfully merged into master.

Build succeeded:

@mathlib-bors mathlib-bors Bot changed the title feat(Topology): An open cover of a preconnected set has no isolated components [Merged by Bors] - feat(Topology): An open cover of a preconnected set has no isolated components May 31, 2026
@mathlib-bors mathlib-bors Bot closed this May 31, 2026
b-mehta pushed a commit to b-mehta/mathlib4 that referenced this pull request Jun 2, 2026
…omponents (leanprover-community#39691)

In this PR we show that a family of open sets whose union is preconnected has the property that for any two (nonempty) sets U and V in the cover there is a sequence of sets in the cover U = U_0, U_1,..., U_n = V such that U_i \cap U_{i+1} is nonempty for all i.

Co-authored-by: Raph-DG <raphaeldouglasgiles@gmail.com>
Bergschaf pushed a commit to Bergschaf/mathlib4 that referenced this pull request Jun 3, 2026
…omponents (leanprover-community#39691)

In this PR we show that a family of open sets whose union is preconnected has the property that for any two (nonempty) sets U and V in the cover there is a sequence of sets in the cover U = U_0, U_1,..., U_n = V such that U_i \cap U_{i+1} is nonempty for all i.

Co-authored-by: Raph-DG <raphaeldouglasgiles@gmail.com>
ReemMelamed pushed a commit to ReemMelamed/mathlib4 that referenced this pull request Jun 20, 2026
…omponents (leanprover-community#39691)

In this PR we show that a family of open sets whose union is preconnected has the property that for any two (nonempty) sets U and V in the cover there is a sequence of sets in the cover U = U_0, U_1,..., U_n = V such that U_i \cap U_{i+1} is nonempty for all i.

Co-authored-by: Raph-DG <raphaeldouglasgiles@gmail.com>
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3 participants