Busy Beaver ones function value $\Sigma(2)$
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
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Lower bound: $4$
Reference unknown
[via Sequence A028444 in the OEIS] -
Upper bound: $4$
Reference unknown
[via Sequence A028444 in the OEIS]