User blog comment:Hyp cos/tree function and TREE(3)/@comment-5150073-20140624084600/@comment-1605058-20140624164456

FBZ, you are right, this is undecidable. Much, much worse, this problem isn't even hyperarithmetic. But we are not talking about that, we are talking about theoretical possibility, e.g. if we knew exactly what machine codes desired ordinal.