This is the current version of a short book about atoms that I am writing. The book begins with a discussion of automata models for infinite alphabets. Then, inspired by these automata, the book develops the theory of orbit-finite sets with atoms and computation on them.

The book was in hiatus for 6 years, but I returned to it in 2025, while giving a course about the topic. The new version has some new material (vector spaces) and deletes some of the automata material from the previous version.

All versions:

• July 3, 2025
• September 11, 2019
• May 22, 2019
• December 28, 2018
• December 4, 2018
• November 30, 2018
• April 15, 2017