Busy Beaver ones function value $\Sigma(2)$

Rule set for 2-state busy beaver

OrdinaryArtery, via Wikimedia Commons. CC BY-SA 4.0.

The maximal number of 1s that a $2$ -state Turing machine can print on an initially blank tape before eventually halting.

Value: $4$

Updates

Lower bound: $4$
Reference unknown
[via Sequence A028444 in the OEIS]
Upper bound: $4$
Reference unknown
[via Sequence A028444 in the OEIS]