Which ordinal indicator, if any, should be used on symbolic math variables?

Consider:

• “the n-th root of x”
• “the a-th derivative of b”

and even more troublesome:

• “the ϑ-th something…” (thetath?)

Is there a rule for which ordinal indicator to use on a name, or symbolic variable?

I have seen “n-th” used in various places, but I cannot say the same for other variable names (x, y, ϑ…)

Is “-th” the preferred indicator for unknown quantities? Is it even acceptable to use one, or should the sentence be rephrased into “the something of order ϑ” whenever possible?

NB. This question was never about the typesetting choice of “n-th” vs. “nth,” or even n^th, but about the use of an ordinal indicator on symbolic variables: short names that are used to represent unknown quantities.

In non-mathematical contexts, such as colloquial use of "the nth degree", "the n^th degree", or "the n-th degree" then all have a justification in analogy to other forms, and either could be used. I'd recommend using the closed form nth only with n because it's more common than other possibilities, which reduces the possibility of confusion. Indeed dictionaries often list nth as a word.

In mathematical contexts there's a possibility of confusion with the superscript form on that it could be read as "to the power of th", that would lead me to favour the hyphenated form.