Add "Used by"

README.md

@@ -25,6 +25,8 @@ Development Status
 
 The fork supports Python 3, increases test coverage, sets up CI and linters, maintains releases to [PyPI](https://pypi.org/project/galgebra/#history), improves [docs](http://galgebra.readthedocs.io) and has many bug fixes, see [Changelog](https://galgebra.readthedocs.io/en/latest/changelog.html).
 
+For information on how GAlgebra is used in other projects, see [Used by](https://github.com/pygae/galgebra/blob/master/doc/used_by.md).
+
 > [!IMPORTANT] 
 > Readers of Prof. Alan Macdonald's [Linear and Geometric Algebra](http://www.faculty.luther.edu/~macdonal/laga/index.html) and [Vector and Geometric Calculus](http://www.faculty.luther.edu/~macdonal/vagc/index.html), please check out [**Migrating guide for readers of LAGA&VAGC**](#migrating-guide-for-readers-of-lagavagc) below.
 > 

doc/used_by.md

@@ -0,0 +1,35 @@
+# Used by
+
+Here is a list of projects that use GAlgebra, divided into two parts:
+
+1. functional: used as a dependency in their major functionality
+2. explorational: used as an optional dependency in their examples, exercises, or tutorials
+
+They are roughly ordered by relevance, stars and last updated time.
+
+This data is manually mined from [forks](https://github.com/pygae/galgebra/forks) and [dependents](https://github.com/pygae/galgebra/network/dependents), as ["Used by" button is not shown](https://docs.github.com/en/code-security/supply-chain-security/understanding-your-software-supply-chain/exploring-the-dependencies-of-a-repository#changing-the-used-by-package). All usages are manually reviewed to rule out false positives (e.g. the dependency maybe added, but is not actually used), some usages are manually added to our knowledge.
+
+## Functional
+
+- Prof. Alan Macdonald's books: [Linear and Geometric Algebra](http://www.faculty.luther.edu/~macdonal/laga/index.html) and [Vector and Geometric Calculus](http://www.faculty.luther.edu/~macdonal/vagc/index.html)
+  - GAlgebra is the books' companion library, see [GAlgebraPrimer](http://www.faculty.luther.edu/~macdonal/GAlgebraPrimer.pdf)
+- [micahscopes/alglbraic](https://github.com/micahscopes/alglbraic): A python library and CLI utility for generating libraries of algebraic GLSL functions, including Clifford algebras
+- [russellgoyder/sundial](https://github.com/russellgoyder/sundial): The sundial problem from a new angle
+  - released as package `analemma` on PyPI
+  - contains a cheat sheet for GAlgebra
+  - see also https://github.com/pygae/galgebra/issues/506
+- [jdekozak/dirac5d](https://github.com/jdekozak/dirac5d): Five dimensional Dirac equation over the reals
+  - see also https://github.com/jdekozak/dirac5d/issues/4
+- [pygae/GAlgebra.jl](https://github.com/pygae/GAlgebra.jl): Julia interface to GAlgebra
+  - contains test cases for [Eric Chisolm's Geometric Algebra](https://arxiv.org/abs/1205.5935)
+
+## Explorational
+
+- Dr. Eric Wieser's PhD thesis *Formalizing Clifford algebras and related constructions in the Lean theorem prover*
+    - in section 3.3. Symbolic, the approach of GAlgebra is demonstrated and discussed
+- [appliedgeometry/poissongeometry](https://github.com/appliedgeometry/poissongeometry): A Python module for (local) Poisson-Nijenhuis calculus on Poisson manifolds, with the following functions
+  - used in notebook tutorials: https://github.com/appliedgeometry/poissongeometry/blob/master/docs/Tutorial_Ingles.ipynb
+- [meuns/galgebra: GA4CS examples, PGA, code-generation, and more](https://github.com/meuns/galgebra)
+  - see also https://github.com/pygae/galgebra/pull/68
+- [hugohadfield/sym_scratch](https://github.com/hugohadfield/sym_scratch)
+  - used in notebooks for CGA