As the article states, it wasn't actually used in the proof where it's credited. Graham later cited it because it was easier to describe than his actual upper b

As the article states, it wasn't actually used in the proof where it's credited. Graham later cited it because it was easier to describe than his actual upper bounds. I'm a big fan of the busy beaver family of problems and the halting problem is particularly interesting. Naturally, the definition of the numbers and thought process used to describe them is orders of magnitude more interesting than the numbers themselves.