User blog comment:MilkyWay90/Help with ordinals/@comment-30754445-20180718030005

You want to try and learn about Madore's Ordinal Collapsing Function.

It is easier to understand than ϑ in my opinion, and it gets you all the way up to the BHO. It can also be extended, quite easily and intuitively, all the way up to a pretty impressive ψ(ψɪ(0)).

(anything beyond that is a whole different kettle'o'fish, though)