BB(5)
BB(5) refers to the 5th value of the Busy Beaver function. In 1989, the 5-state busy beaver winner was found: a 5-state Turing machine halting after 47,176,870 giving the lower bound BB(5) > 47,176,870. In 2024, BB(5) = 47,176,870 was proven by the bbchallenge.org massively collaborative research project.