I'm charged with translating a technical document into English, and ran into a bit of an odd problem: the document refers to undefined numbers of elements, and uses letters to represent those numbers, as you would see in algebra. The order of those elements are important, so they need to be written as ordinal numbers.
In the source language (Japanese), an ordinal number is denoted with the character "第" (dai), so "第1" becomes "first", "第2" becomes "second", and so on. But how should I translate "第s"? "Nth" is common in English to refer to an undefined degree, but "rth" or "sth" seems confusing to me. Unfortunately, uppercase and lowercase letters are defined as different numbers, so I can't simply capitalize.
Is there a convention for this? Should I add a hyphen e.g. "s-th", or enclose the letter in parenthesis e.g. "(s)th"?