[Merged by Bors] - feat(RingTheory/RamificationInertia/Inertia): inertia degree is invariant under a group action

[tb65536]

This PR proves that inertia degree is invariant under a group action. We already have this for the old definition inertiaDeg, but we will need this new version for inertiaDeg' for the upcoming refactor.

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• depends on: [Merged by Bors] - feat(RingTheory/Localization/AtPrime/Basic): add variants of localAlgHom and localAlgEquiv #39714

[github-actions]

PR summary 9a8635f65b

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff (regex)

+ Ideal.residueFieldAlgEquiv
+ Ideal.residueFieldAlgEquiv'
+ Ideal.residueFieldRingEquiv
+ inertiaDeg'_smul
+ instance : IsLocalHom (algebraMap (Localization.AtPrime I) (Localization.AtPrime J)) := by
- instance (p : Ideal R) [p.IsPrime] (q : Ideal A) [q.IsPrime] [q.LiesOver p]

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.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit 9a8635f).

• +5 new declarations
• −1 removed declarations

+Ideal.inertiaDeg'_smul
+Ideal.residueFieldAlgEquiv
+Ideal.residueFieldAlgEquiv'
+Ideal.residueFieldRingEquiv
+instIsLocalHomAtPrimeRingHomAlgebraMap
-instIsLocalHomAtPrimeRingHomAlgebraMapOfIsLiesOverAlgebra

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit 9a8635f65b
Reference commit eb15debe77

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/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).

[mathlib-dependent-issues]

This PR/issue depends on:

• [Merged by Bors] - feat(RingTheory/Localization/AtPrime/Basic): add variants of localAlgHom and localAlgEquiv #39714
  By Dependent Issues (🤖). Happy coding!

[riccardobrasca]

Thanks!

bors merge

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

• ci (staging) / Post-CI job
• ci (staging) / Lint style
• ci (staging) / Build
• ci (staging) / Post-Build Step