User blog comment:Ikosarakt1/Fast-growing hierarchy/@comment-78.22.170.27-20130619192628/@comment-78.21.60.230-20130629120609

The Gordeev style notations can be found in

http://link.springer.com/article/10.1007%2FBF01630809#page-1

The approach is very elegant. An interpretation in the $\psi$-functions is not totally obvious.

Best,

Andreas Weiermann