User blog comment:Ikosarakt1/Extension of Chris Bird's array notation./@comment-1605058-20130206143113/@comment-5150073-20130206145805

I firstly heard about ϑ function from Chris Bird himself. This is not seems that ordinals such Ω_(ω+1), Ω_(ω+2), Ω_Ω, Ω_Ω_Ω can't be defined using ϑ. You can read something about it in wikipedia.