I will take this opportunity to promote the YouTube channels of Brady Haran[0], Numberphile specifically. It has all sorts of interesting videos, including Graham's number [1] (one of the videos featuring Ron Graham) and TREE(3) [2].
His other channels are worth checking out too, Sixty Symbols and Deep Sky Videos being my favorites. To be clear, I have no affiliation with this, just a fan.
[1] https://www.youtube.com/watch?v=HX8bihEe3nA&list=PLt5AfwLFPx...