For example, the number 9 can be found by multiplying 3 by 3, and the number 12. You get it by multiplying 3, 2 and 2.
All natural numbers (greater than 1) can be put in one of the two classes below:
- The number is prime.
- The number is not prime. It can be found by multiplying together other primes. The same prime number can be used several times, as in the example with 12 above. This is known as the fundamental theorem of arithmetic.