We can write a set either by listing its elements or describing the set. Consider the set of prime numbers less than 20 to be denoted as A, therefore we can represent the set A as A= (2, 3, 5, 7, 11, 13, 17, 19), A= (x/x is a prime number less than 20). I mathematics, we demote a set by upper case letters while its elements by lower case letters. For example, the set of natural numbers is denoted by N.We can write a set either by listing its elements or describing the set. Consider the set of prime numbers less than 20 to be denoted as A, therefore we can represent the set A as A= (2, 3, 5, 7, 11, 13, 17, 19), A= (x/x is a prime number less than 20). I mathematics, we demote a set by upper case letters while its elements by lower case letters. For example, the set of natural numbers is denoted by N.