The original lesson text and illustration sources are dedicated to the public domain under CC0 1.0.
https://creativecommons.org/publicdomain/zero/1.0/

Human reference works retain their own rights; citations do not dedicate those works to CC0. Third-party source PDFs are not included. MathJax-generated mathematical glyphs accompany the reader; its Apache 2.0 notice is in licenses/MathJax-LICENSE.txt.