If number of elements in set is 0, it is an empty set.

It is also called null set or void set.

Example:

## A = {x: x is a natural number less than 1}

Since, natural number = 1, 2, 3, 4, …

There is no natural number less than 1

Hence,

A = {}

or A is an empty set.

We can also write

A = ∅

##
B = {x: x
^{
2
}
= 4, x is an odd number}

x
^{
2
}
= 4

x = ± √4

x = ± 2

So, x = -2, 2

Since, x is even, it means set B is empty

B = {} = ∅