nought
/nɔːt/
noun
- Nothing; something which does not exist.
- A thing or person of no worth or value; nil.
- Not any quantity of number; zero; the score of no points in a game.
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If you were reading it out to somebody, eg. to write down in a lecture, I would say N-sub-zero. But if this is a particular mathematical term then it's probably N-nought, or N-null. Sometimes the term has a particular usage from history or convention (eg the original paper or a famous textbook) whatever the rules of regular English grammar might say.
eg. the set of cardinal numbers is normally Aleph-nought or Aleph-null
From a native US English speaker:
As there don't seem to be any readily available pronunciation guidance resources on this subject, I am forced to be subjective. I believe "zero" is the most common, in the US, at least. Since I do not prefer to call "0" anything other than "zero" in any situation, I would say:
N-zero for nₒ
N-two for n₂
Etc.
As to your last question, I confess I wouldn't understand someone who said "N-nor", without further enlightenment.