# Apostrophe to indicate the plural of a mathematical object [duplicate]

I have a question on the use of apostrophe to indicate the plural of a mathematical object.

Consider the sentence: "There are many values of X such that the statement is true".

In math, often we re-write this sentence as "There are many X's such that the statement is true".

Now, my question is: which one is correct (or preferred) between

"There are many X's such that the statement is true"

and

"There are many Xs such that the statement is true"

In case they are both wrong, please explain.

• While sometimes adding 's can be acceptable for plurals of single letters (see, for instance answers to this question), particularly in mathematics, where it is common to use apostrophes/primes to denote other variables (e.g. X, X', X'' etc.), or for differentiation, it would be liable to cause confusion. – TripeHound Aug 1 '19 at 11:21