The main question here is whether using -(e)th to create ordinals out of cardinals1 is still considered a productive suffix in English. Is it?
If so, then does it matter whether we are in a formal or informal register? Is it accepted in some contexts but not in others? Is it blocked by certain circumstances?
Consider these sorts of formations, arranged by least to most shocking; are they legal, illegal, or suspect?
- the nth or nth item
- the zeroth element
- googolth for which item is number googol
- epsilonth and ¹⁄epsilonᵗʰ (¹⁄epsilonth)
- alphath or perhaps alphath
- calculating the (1−alpha)th percentile
- three-halfth for ³⁄₂ᵗʰ (³⁄₂th)
- two-thirdth for ²⁄₃ᵗʰ (²⁄₃th
- Such as we do with four > fourth, fifty > fiftieth, and billion > billionth