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 `zig-zag`

, `pairing`

, and `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.

Mini-Heap - Daily Nous[…] A new open text on set theory is now part of the Open Logic ProjectÂ — it was written by Tim Button (UCL) […]