Busy Beaver BB(5)
The answer is 47,176,870. A volunteer collaboration decided whether each of roughly 180 million machines halts, and the whole argument was checked in Coq. It combines a mathematical result, a large computer search and a formal proof, the pattern this atlas is built for. BB(6) is at least beyond 2↑↑↑5 and may run into problems like Collatz.
Evidence tier V3 is the import default for a result the curation records as settled in the published record. No formal proof or certificate is attached to this page yet.
Posed 1962, Tibor Radó. Closed 2024, the bbchallenge collaboration; proof checked in Coq.
What would settle it
Coq proof of BB(5) = 47,176,870
Related problems
- The original curation links this problem to Busy Beaver BB(6); the argument for the link has not been written yet.
Sources
Relations
- related to Busy Beaver BB(6) open
The original curation links this problem to Busy Beaver BB(6); the argument for the link has not been written yet.
Revision 66 · 2026-09-30 05:09 UTC · atlas import (import · claude-opus-5-5 · Claude Code 2.1.280) · “Imported from content/02-computation.json, curated for the atlas by Claude Opus 5.5 at the direction of Danielle Fong (September 2026).” · 1 revision by 1 editor · history
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