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Which of the following is correct:

  • ... where c is a constant, f(.) is a monotonic function, x and y are random variable.
  • ... where c is a constant, f(.) is a monotonic function and x and y are random variable.

This always gets me confused. Because x and y are of one kind (I don’t know if that is the proper word) and there should be an ‘and’ in between, but I’m not about whether not I should write an ‘and’ before the phrase too because it is the last item in a list (I mean it’s like saying: I bought apples, oranges, and bananas.).

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    Although not everyone will agree with me, I think I would edit this to read "...where c is a constant, f(.) is a monotonic function, and x and y are random variables."
    – Steven Venti
    Commented Sep 24, 2018 at 8:02
  • I agree with @StevenVenti, I’d include that final "and" for my own grammatical consistency. But I’ve seen and understood both, so there’s no harm in leaving it out either.
    – Pam
    Commented Sep 24, 2018 at 12:02
  • @AndyT so you’re saying the language in math writing is not proper English? I don’t see how the question is mathematically based.
    – Lod
    Commented Sep 26, 2018 at 14:47
  • @Lod - I meant I couldn't understand it with the "c" "f(.)" etc in it. The mathematical notation broke up my thought process. Re-reading it now, I can see the language aspect in it. Close vote retracted.
    – AndyT
    Commented Sep 26, 2018 at 15:12
  • See also Which punctuation is correct in this case (colon, semicolon or period)?
    – Scott - Слава Україні
    Commented Sep 26, 2018 at 17:53

2 Answers 2

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Yes, there should be "and x and y" to account for the last "and" that should go at the end of a list.

Her favorite types of sandwiches are ham, turkey, and peanut butter and jelly.

If you do not like that, you could rearrange the items in the list to avoid having to say "and x and y."

Her favorite types of sandwiches are peanut butter and jelly, ham, and turkey.

It is not always incorrect to omit the last "and" in a list of items, though; there is a literary technique called asyndeton that involves just that. However, it's more of a creative technique rather than something you should use in, say, a math paper.

When she arrived at the market, she stared in awe at everything that was in stock: swords, crystals, gems, spices from faraway lands.

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  • In the asyndeton example, the absence of and signifies that it is not an exhaustive list. It's not an optional omission.
    – Kris
    Commented Sep 24, 2018 at 8:12
  • @Kris that doesn't sound correct: it has nothing to do with whether the list is exhaustive or not. The omission is entirely optional as a literary technique, exactly as Tommy has shown in the final block quote.
    – Chappo Hasn't Forgotten
    Commented Sep 24, 2018 at 13:30
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It's correct to add the extra and, but not for the reason you've given. There's a difference in grammar between the suggested extra and, and the and in "x and y".

In the latter, and joins two nouns, whereas the "extra" and is used to join two clauses. It is not only permissible to use and in both circumstances, it's preferable to do so, for two reasons: it's grammatically correct, and it reduces ambiguity for the audience. The correct expression would therefore be:

... where c is a constant, f(.) is a monotonic function, and x and y are random variables.

Note that the first and is preceded by a comma, and that we use the plural variables (as there are two of them, x and y).

For further reference, see:

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  • Conjunctions do not necessarily join nouns or clauses alone. The comma before and has a long story, it's not to be trivialized with "Note that the first and is preceded by a comma." See previous related posts here.
    – Kris
    Commented Sep 27, 2018 at 8:47

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