GeoGebra (https://www.geogebra.org) is a set of open source math apps for instance for geometry, graphing, 3D graphing, CAS and other purposes (GeoGebra Graphing Calculator, GeoGebra Geometry, GeoGebra 3D Graphing Calculator, GeoGebra Classic 6, GeoGebra Exam Simple Calculator, GeoGebra Exam Graphing Calc, GeoGebra Exam CAS Calculator). It is also available for Apple and PC platforms.
According to https://www.geogebra.org/license the prebuilt apps are free for non-commercial use only, because of a non-libre licence of the installer component, but the GeoGebra code itself is licensed under the GPL.
The sources are available here: https://dev.geogebra.org/trac/browser
Would GeoGebra qualify for F-Droid?