Prime and coprime numbers

Prime number is a positive natural number that has only two positive natural divisors: one and itself.

The opposite of prime numbers are composite numbers. Composite Number is a positive natural number that has at least one positive divisor other than one or itself.

Coprime numbers are numbers A and B that do not have any common divisors, except for one.

Examples of prime, composite, and coprime numbers

1. The number 2 is a prime number, because has only two divisors – 1 and 2:
   • 2 / 1 = 2
   • 2 / 2 = 1
2. Number 15 is not a prime number, because it has divisors – 1, 3, 5, 15:
   • 15 / 1 = 15
   • 15 / 3 = 5
   • 15 / 5 = 3
   • 15 / 15 = 1
3. Number 13 is a prime number, because has only two divisors – 1 and 13:
   • 13 / 1 = 13
   • 13 / 13 = 1
4. The numbers 2 and 5 are coprime, because have only one common divisor – the number 1:
   1. 2/1 = 2
   2. 2 / 2 = 1
   3. 5/1 = 5
   4. 5 / 5 = 1

Prime number table

Below is a table of prime numbers from 2 to 997, which you can print out so that you always have it on hand if necessary.

Questions – answers

1. Is the number 1 prime?

   Number 1 is not prime by definition – it has only one divisor.

2. Is the number 0 prime?

   Number 0 is not prime – it is not a positive number and has an infinite number of divisors.