"The roots could be any arbitrary distance apart."
"The roots could be an arbitrary distance apart."
Using "any" is more redundant as I could take out "arbitrary" and it would still make sense (although it wouldn't quite convey the meaning I want), but it flows better than "an". Hence, would it be better to use "any" for the sake of flow or "an" for the sake of non-redundancy?
Any
andan
are two completely different words, so it's hard, or just odd, to compare their usage. Explain what you mean to convey and that might help.