Busy Beaver number $BB(2)$
OrdinaryArtery, via Wikimedia Commons. CC BY-SA 4.0.
The maximal number of steps that a $2$-state Turing machine can make on an initially blank tape before eventually halting.
Value: $6$
Updates
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Lower bound: $6$
Reference unknown
[via Sequence A060843 in the OEIS] -
Upper bound: $6$
Reference unknown
[via Sequence A060843 in the OEIS]