TMBR: August 2025

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This Month in Beaver Research for August 2025, featuring plenty of holdouts reduction in numerous domains, alongside an upgraded TM-visualizer and a couple new BB adjacent games.

@mxdys Introduced "50 Random Holdouts", a thread on the Discord server, where 50 random TMs are selected from the BB(6) holdout list, and everybody focuses on these 50 machines. This month, 6/50 TMs were solved by @mxdys single-handedly.

Cryptids

A fast algorithm for Consistent Collatz simulation was re-discovered and popularized. Using it:
- apgroucher simulated Antihydra to $2^{38}$ iterations. This is actually a result from one year ago, but was rediscovered and added to the wiki. https://discord.com/channels/960643023006490684/1026577255754903572/1271528180246773883
- Shawn Ligocki simulated 1RB1RA_0RC1RC_1LD0LF_0LE1LE_1RA0LB_---0LC (bbch) out to one additional Collatz reset, demonstrating that (if they halt, which they probviously should) they will have sigma scores $>10^{10^{10^{7}}}$ .

Holdouts

BB(6) holdouts: Reduced by a total of 26 holdouts by 4 people.
- XnoobSpeakable found 9 new halting TMs in the high exponential runtime range (~ $10^{100000}$ ) by running Enumerate.py out to extremely high parameters. https://discord.com/channels/960643023006490684/1239205785913790465/1401470301467836556
- Andrew Ducharme found a surprisingly short running halting TM in the BB(6) holdouts list with runtime ~ $10^{78}$ , to which Peacemaker replied with another TM that was almost identical, and soon, simulation showed it to halt in the same number of steps. Later on the 28th, Ducharme found another one with surprisingly low runtime: ~ $10^{11}$ . In response, Peacemaker found an almost identical machine, which also halts with similar runtime.
- Peacemaker shared a list of BB(6) holdouts and how many steps are required to use all defined transitions. https://discord.com/channels/960643023006490684/1239205785913790465/1410437756777398344
- @mxdys shared a list of 7 holdouts that he solved using his RWLAcc decider in Rocq (previously known as Coq). https://discord.com/channels/960643023006490684/1239205785913790465/1408304281039409212 He also shared results featuring 6 holdouts that were solved in "50 Random Holdouts", see https://discord.com/channels/960643023006490684/1400456788955893840/1409115537631613020
After the enumeration of BB(7) was completed, Andrew Ducharme ran several deciders on the holdouts list, filtering the original 86,129,304 holdouts down to 60,765,943 in 8 days. https://drive.google.com/drive/u/0/folders/17U0BRpJHTMLtB0poBlOSZhGGp4FkCHIO
BB(3,3): 9 holdouts were proven non-halting in Rocq (previously known as Coq) by mxdys. 10 holdouts remain, 4 of them solved with moderate rigor. https://discord.com/channels/960643023006490684/1259770474897080380/1410308974275985428

BB Adjacent

John Tromp introduced the $BB\lambda _{1}(n)$ function for Busy Beaver for lambda calculus with an oracle and computed it up to $BB\lambda _{1}(22)$ .
Instruction-Limited Greedy Busy Beaver gBBi(n) and an Instruction-Limited variant of the Blanking Busy Beaver (BLBi(n)) were introduced. gBBi(n) was computed up to n = 13 and BLBi(n) was computed up to n = 7.

Misc

Iijil shared an algorithm for converting an arbitrary n-state m-symbol TM into a 2-state TM with 3(n+1)m symbols. https://gist.github.com/Iijil1/0d611dbf0a9d52984f72cb14e66a4b28
Carl K updated his TM web-visualizer to support multi-symbol machines. https://carlkcarlk.github.io/busy_beaver_blaze/v0.2.6/index.html He also extended his series of videos showing TM simulation accompanied by classical music out to some multi-symbol TMs:

In the News

22 Aug 2025. Ben Brubaker. Quanta Magazine. Busy Beaver Hunters Reach Numbers That Overwhelm Ordinary Math.

Interesting TMs

A collection of interesting TMs that were mentioned on Discord, mostly because of their space-time diagrams or general behavior.

1RB0LE_1RC0RF_1RD---_0LA1RB_1RB1LE_1LD1RF (bbch): Wavy Machine