User blog comment:B1mb0w/The Alpha Function/@comment-27173506-20160129134940/@comment-27513631-20160201160943

$$TREE(3)>f_{SVO+2}(f_{SVO+1}(f_{SVO}(6)))$$ should be a valid bound. (Given that $$tree(k)\approxf_{SVO}(k)$$.)

Source: http://math.stackexchange.com/questions/313134/how-does-tree3-grow-to-get-so-big-need-laymen-explanation