feat: add formula for the j-invariant of an elliptic curve

[Paul-Lez]

This doesn't seem to be in Mathlib yet (?).

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[github-actions]

PR summary ce9a86929f

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)

+ j_eq

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 ce9a869).

• +1 new declarations
• −0 removed declarations

+WeierstrassCurve.j_eq

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit ce9a86929f
Reference commit 6f60478685

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).

[Multramate]

Make this a lemma. Surely K and W are already defined elsewhere. Can you also move this up to right below the definition?

[Paul-Lez]

K and W are defined elsewhere (R instead of K, but as a CommRing so can't be used directly).

[Multramate]

Ah gotcha, this is one of the few stupid files where we don't have W' for R and W for K...