Tim Button (University College London) based an entire (open) textbook on set theory on the existing set theory coverage of the Open Logic Project, and agreed to have his changes and additions included in the main repository. The material from his Open Set Theory is now folded into the
content directory of the OLP. There is also a new textbook, closely resembling the original Open Set Theory. The PDF of Set Theory: An Open Introduction can be found, like all the others, on builds.openlogicproject.org.
Incorporating Open Set Theory into the OLP structure required a few changes to existing material. The coverage of naive set theory in
content/sets-functions-relations is now extended by two additional chapters, but in order to fit with later developments, some of the material in the
size-of-sets chapter now comes in two versions. I’ve tried to make these changes as much backwards compatible as possible, but if you are including any of this material in your own remixes, you might see some odd results after updating to the new version. Specifically, there is now a new
relations/equivalence-relations section (split off from special-properties), and the long section
size-of-sets/enumerability has been split, with new separate sections
pairing-alt. The last two sections are rewritten.
The other material from Open Set Theory now mainly lives in a new
content/set-theory part and a
content/history/set-theory chapter. The material has been changed minimally, mainly to conform to spelling and stylistic conventions in the rest of the OLP.