BB(2,7)
The 2-state, 7-symbol Busy Beaver problem, BB(2,7), is unsolved. With Cryptids like Hydra in the preceding domain BB(2,5), we know that we must solve a Collatz-like problem in order to solve BB(2,7). The current BB(2,7) champion is simply inherited from BB(2,6) as BB(2,7) is just starting to be explored.
Top Halters
These are the 20 highest scoring machines that were found in the current investigation of BB(2,7). This list does not include TMs from smaller domains:
Phase 1
The initial phase of enumeration and reduction of holdouts started in November 2025 using C++ code mxdys shared. This work is ongoing and further described below. After this is completed, it is planned to run the Ligockis' Enumerate.py code to further reduce the number of holdout TMs.
Stage 1
The code provided by mxdys breaks up the BB(2,7) enumeration into 1 million subtasks which each run for ~10 minutes and leave ~2500 holdouts based on an average of the first 1K subtasks. These values are about 5 times longer than and 25 times larger than the ones for BB(7).
This enumeration began in November 2025 and is summarized in the table below. The mxdys code main.cpp from C++ code was used with two modification: #define BB7 was replaced with #define BB27 and the following block of code was added in the section with BB domain definitions:
#ifdef BB27 constexpr int64_t N_STATE=2; constexpr int64_t N_CHAR=7; constexpr int64_t log2_N_STATE=1; constexpr int64_t log2_N_CHAR=3; typedef uint8_t chr_t; #endif
Results:
| Task range | Done by | Completed | # holdouts | Source |
|---|---|---|---|---|
| 00-09xxxx | Terry Ligocki | Yes | 309,868,865 | Google Drive |
| 10-19xxxx | Yes | 315,243,072 | ||
| 20-29xxxx | Yes | 309,968,380 | ||
| 30xxxx | Yes | 35,062,963 | ||
| 31xxxx | No | TBD | ||
| 32xxxx | No | TBD | ||
| 33-36xxxx | TBD | No | TBD | --- |
| 37xxxx | Terry Ligocki | No | TBD | Google Drive |
| 38xxxx | No | TBD | ||
| 39xxxx | Yes | 27,889,734 | ||
| 40xxxx | Yes | 26,746,871 | ||
| 41-99xxxx | TBD | No | TBD | --- |
| Cumulative | --- | --- | 1,024,779,885 | --- |
Stage 2
The Ligocki's "Enumerate.py" Python code is used to further reduce the number of holdouts and explore more of the TNF of BB(2,7).