Busy Beaver number $BB(4)$
OrdinaryArtery, via Wikimedia Commons. CC BY-SA 4.0.
The maximal number of steps that a $4$-state Turing machine can make on an initially blank tape before eventually halting.
Value: $107$
Updates
-
Lower bound: $107$
Reference unknown
[via Sequence A060843 in the OEIS] -
1983-04
Upper bound: $107$
Brady, A. H. (1983). The determination of the value of Rado's noncomputable function Σ(k) for four-state Turing machines. Mathematics of Computation, 40(162), 647-665.
[via Sequence A060843 in the OEIS]