BB(5) paper accepted to STOC 2026

I’m very happy to share that our paper, Determination of the fifth Busy Beaver value, has been accepted to STOC 2026: I will be presenting the paper in Salt Lake City this June!

Over three years in the making: from launching the Busy Beaver Challenge in March 2022, to announcing the Coq proof that BB(5) = 47,176,870 in July 2024, to writing the paper over the past year.

The paper is a “human-readable” version of the Coq proof, describing how we enumerated more than 180M Turing machines and, for each, decided whether it halted or not. If you want to learn more about the project, check out Ben Brubaker’s Quanta Magazine article or the associated YouTube video.

Many thanks to all those who contributed to this result and to bbchallenge in general, past and present. Research on BB(6) and beyond is still ongoing, feel free to join our Discord if you’re interested!