[Merged by Bors] - feat(NumberTheory.Height.Northcott): lemmas on compositions

[MichaelStollBayreuth]

This adds two lemmas to Mathlib.NumberTheory.Height.Northcott:

• Northcott.lemma comp_of_bddAbove: the post-composition of a Northcott function with a function that pulls back bounded above sets to bounded above sets is Northcott
• Northcott.comp_of_finite_fibers: the pre-composition of a Northcott function with a function that has finite fibers is Northcott.

They will be useful in the context of heights.

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[github-actions]

PR summary 900c9b0390

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

+ comp_of_bddAbove
+ comp_of_finite_fibers
+ exists_min_image
- Northcott.exists_min_image

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.

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No changes to technical debt.

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).

[tb65536]

Thanks!

maintainer merge

[github-actions]

🚀 Pull request has been placed on the maintainer queue by tb65536.

[riccardobrasca]

Thanks!

bors merge

[mathlib-bors]

Pull request successfully merged into master.

Build succeeded:

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