Busy Beaver ones function value $\Sigma(4)$
OrdinaryArtery, via Wikimedia Commons. CC BY-SA 4.0.
The maximal number of 1s that a $4$-state Turing machine can print on an initially blank tape before eventually halting.
Value: $13$
Updates
-
Lower bound: $13$
Reference unknown
[via Sequence A028444 in the OEIS] -
1983-04
Upper bound: $13$
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 A028444 in the OEIS]