Question: what is xn in math?
Answer: Set membership x ∈ X means x is an element of the set X. ... Listed sets {x1,x2,...,xn} denotes the set whose elements are x1,x2,...,xn (n ≥ 1); in the case n = 1 we get the singleton set {x}, whose unique element is x. Empty set ∅ denotes the set containing no elements; it is sometimes written as {}.