Find the missing number?

2
0
3
1

The answer is 1.

The numbers in the sequence are all prime numbers, except for 1. 1 is not a prime number, because it is divisible by itself and by 1. All other numbers in the sequence are only divisible by themselves and by 1.

A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number.