User talk:Binary198/List of ordinal collapsing functions

Nice list! There was also another OCF by Arai that I saw that had an interesting property. Some simpler OCFs, such as Bachmann's ψ and Buchholz's ψ, are Σ1-definable (source: personal communication, although I don't know for stronger OCFs like Rathjen's). But this OCF is not Σ1-definable, in fact it's not even Σn-definable for any constant n C7XWiki (talk) 02:01, 3 September 2021 (UTC)