grammar - The use of ‘and’ after ‘where’ in a mathematical statement

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.).

Answer

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.