TMBR: September 2025

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This Month in Beaver Research

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This Month in Beaver Research for September 2025.

TODO: BB(5) arXiv released https://arxiv.org/abs/2509.12337

TODO: BB(3,3) month next month.

In the News

• 14 Sep 2025. Ben Brubaker. Wired. The Quest to Find the Longest-Running Simple Computer Program.
• 17 Sep 2025. Hacker News. Determination of the fifth Busy Beaver value.
• 18 Sep 2025. Tuomas Kangasniemi. Tekniikkatalous. Iso matematiikan ongelma ratkesi 63 v jälkeen (Finnish) (English: A big math problem solved after 63 years).

Blog Posts

• 12 Sep 2025. Katelyn Doucette. Bugs, Mazes, and the Unreasonably Effective Brady's Algorithm.
• 23 Sep 2025. Katelyn Doucette. Building the Busy Beaver Ladder.

Holdouts

This month saw huge reductions to holdout lists in many domains. In BB(6), this was mainly due to mxdys demonstrating the equivalence of many TMs. For the other domains it seems to be mainly due to applying mxdys's main.exe decider pipeline to these domains.

BB Holdout Reduction by Domain

Domain	New Holdout Count	Aug Holdout Count	Holdout Reduction	% Reduction
BB(6)	1,691	2,592
BB(7)	23,314,388?	59,727,905
BB(2,6)	873,469?	18,054,938
BB(4,3)	97,701,052	460,916,384

• BB(6):
  • Andrew Ducharme and @mxdys both found a family of 10 halting TMs independently, all halting in around $10^{69}$ steps.
  • @mxdys decided two machines’ fates from the 50 Random Holdouts released in August, making 8/50 machined solved as of this month.
  • Andrew Ducharme found a non-halting TM, to which Peacemaker II replied with another machine that turned out to be very similar, and non-halting as well. Later in his search Andrew found another non-halting TM.
  • @mxdys shared a new way of grouping TMs by equivalence class, this reduced the number of holdout equivalence classes from 2467 to 1691 (31% reduction).
  • @mxdys published a list of 3171 holdouts. The holdouts are in 1691 equivalence classes by halting status.
• BB(7):
  • Andrew Ducharme has continued reducing the number of holdouts, from 59,727,905 to 28,189,617 (52.80% reduction).
  • Terry Ligocki ran an additional 41 filters/parameters. This reduced the number of holdouts, from 28,189,617 to 23,314,388 TMs (17.29% reduction)
  • The overall reduction this month was 72.9% (note: This point can be discarded after the table is made for overall reduction for all domains)
• BB(2,6):
  • TODO: Add events before @Peacemaker II noticing the errors. (i.e. Terry ran the filters on the wrong holdout list etc..)
  • @Peacemaker II noticed some TMs missing from Andrew Ducharme reductions. It was decided to start back at the original 22,302,296 holdout TMs. (TODO: Source)
  • Andrew Ducharme ran several deciders/filters and reduced the 22,302,296 holdout TMs to 20,358,011 (8.72% reduction). (TODO: Source)
  • Terry Ligocki ran 50 variations of deciders/parameters using @mxdys' C++ code, main.cpp, reducing the holdout count to 970,101 TMs (95.23% reduction)!
  • Andrew Ducharme ran some additional deciders reducing the holdout count to 873,469 TMs (9.96% reduction). (TODO: Link to wiki page where you can find the specific filters)
• BB(4,3):
  • Terry Ligocki ran a set of deciders/parameters from @mxdys' code to reduce the number of holdouts which were at 460,916,384 TMs. Six passes have reduced the holdout count to 97,701,052 TMs (78.80% reduction). (TODO: Source)
• BB(3,4):
  • TODO: XnoobSpeakable and Lúkos are working on the holdout list for BB(3,4), ~435M TMs, using @mxdys' deciders with various parameters.

BB Adjacent

• John Tromp announced on Discord that a 350-bit $BB\lambda (n)$ function now reaches the limit of BMS, an improvement from the previous 404 bits.

TODO: phrasing. Discord source: https://discord.com/channels/960643023006490684/1355653587824283678/1413637783045542038 and https://github.com/tromp/AIT/blob/master/fast_growing_and_conjectures/bms.lam

• TODO: savask shared "Busy Bug Game": https://discord.com/channels/960643023006490684/1362008236118511758/1415723582989930679

Misc

• TODO: Affine maps are Turing complete and its relation to BMO1: https://discord.com/channels/960643023006490684/1239205785913790465/1420457986564030641 https://discord.com/channels/960643023006490684/1239205785913790465/1420491357969059910