# In which cases the subscript is a "0" (zero) and an "o" (letter o)?

As stated in the title, the question is simple, about formatting. Reading through physics papers, I have seen $$\varepsilon_o$$, $$\rho_o$$, $$\mu_o$$, to denote some physical constants. Note that all of them use the "$$_o$$" subscript (lower case o), but other autors use a "$$_0$$" subscript (number zero), for example, in $$\aleph_0$$.

Is there a consensus about this notation? Which one should be used? Are they both preferred depending on the field?

P.D.: I am spanish, so maybe I am not using the exact or correct terms to ask this question. I hope it is understandable.

• I have seen $\epsilon_o$ with power point. In LateX then the same authors wrote $\epsilon_0$. So I suppose that often just zero $0$ is intended for the index. Commented Jun 4, 2023 at 9:49

• The numeral $$0$$ is used when whatever your using it on is the initial something: $$\aleph_0$$ is the initial infinite cardinal, $$a_0$$ is a very common generic name for the initial term of a sequence, and so on (also if it's not initial, but the number $$0$$ is still relevant to the specific object, either through construction or through meaning)
• The letter o signifies a word that begins with o, or an already established object denoted by $$o$$: given a set $$X$$ of integers, if you want to split it into subsets according to parity, it is common to let $$X_o$$ signify the odd subset
Generally, I would say the numeral $$0$$ is a lot more common.